Communication management method, communication terminal, communication management program, recording medium containing the communication management program, and communication system

ABSTRACT

When a transmission station having a voluntary transmission right transmits FINALL FRAME as a TXOP ending frame to HC, the HC transmits an acknowledgement frame ACK after SIFS following the reception of FINAL FRAME. Here, when the transmitting station has detected that no frame transmission is performed over the communication network from any other ESTAs in PIFS after the transmission of FINAL FRAME, FINAL FRAME is retransmitted in DIFS after the transmission of FINAL FRAME. By performing such a communication management, it is possible to accurately manage a voluntary transmission right even when the communication medium has a low reliability in a network in which one network pathway is time-shared by a plurality of communication apparatuses.

TECHNICAL FIELD

The present invention relates to a communication management method in anetwork in which one network pathway is time-shared by a plurality ofcommunication apparatuses, and especially relates to the IEEE 802.11wireless communication standards.

RELATED ART

Conventionally, in a computer network and the like, frame transmissionand reception are performed in accordance with a communication systemcalled “packet communication system”. Nowadays, there is increasingdemand for construction of a wireless network in home-use LAN (LocalArea Network), for example. Such a wireless LAN, as compared to a wiredLAN, has the advantages of eliminating the need for wiring such ascabling and increasing mobility of a device connected to the LAN.

In a network including such a wireless LAN, a plurality of communicationapparatuses connected to the network share by time one network pathwayfor frame transmission and reception. In such a system, efficiency ofbandwidth use changes significantly depending on a management method fora transmission right.

For example, it is considered that a scheme for bandwidth management ofa network pathway called “HCF” (Hybrid Coordination Function incompliance with the IEEE Std 802.11e/D1.2, July 2001) is added to thespecifications of the IEEE 802.11 wireless LAN communication standards(in compliance with ANSI/IEEE Std. 802.11, 1999 Edition).

In the HCF, it is assumed that a central management device called HC(Hybrid Coordinator) manages transmission rights of all communicationapparatuses belonging to a network. Here, among the communicationapparatuses belonging to the network, a communication apparatus otherthan the HC is referred to as ESTA (Enhanced Station).

The HCF is a scheme developed for transmission of realtime data such asvideo data. In this HCF, each ESTA reports to the HC information onproperties of data to be transmitted from itself. As used herein,“information on properties of data” refers to information on how muchvolume of data is to be transmitted at what time intervals. The HChaving received such information from the ESTAs schedules the order ofgranting a voluntary transmission right and a grant duration of thevoluntary transmission right so as to meet the requests from the ESTAs,and the voluntary transmission right is granted to each of the ESTAs inaccordance with this schedule.

The HC grants the voluntary transmission right with time limit called“TXOP” (Transmission Opportunity) to the ESTA. Then, in order to notifythe grant, the HC transmits a frame called “QoS CF-POLL” to the ESTAwhich the HC will granted the voluntary transmission right to. QoSCF-POLL includes information on grant duration of the voluntarytransmission right, which is called “TXOP LIMIT.” The ESTA to which QoSCF-POLL is addressed is allowed to transmit data during this duration.Hereinafter, an ESTA having been granted the TXOP is called “TXOPholder”.

Each of the ESTAs has a value, called “NAV” (Network Allocation Vector),to manage whether the ESTA itself is granted the voluntary transmissionright. When the NAV value is more than 0, the ESTA is only allowed tomake a response to a frame transmitted from another ESTA (for example,upon reception of data frame, to return a transmission acknowledgementframe) and is set not to initiate data transmission from itself.

An ESTA to which QoS CF-POLL is not addressed, when receiving QoSCF-POLL, sets a value indicated by TXOP LIMIT to NAV. The NAV valuedecreases with the passage of time, and when the NAV value decreases to0, the ESTA recognizes that it has been allowed a transmission, andattempts a data transmission from itself by a scheme called “DCF(Distributed Coordination Function).

In the DCF, when a wireless medium is detected idle for a period called“DIFS” (Distributed Coordination Function Inter Frame Space), each ofthe ESTAs initiates a count-down timer called backoff timer. Thisbackoff timer is a count-down timer that the ESTA starts from a randomvalue in a predetermined range. If the wireless medium is idle when thebackoff timer becomes zero, the ESTA obtains the voluntary transmissionright and initiates data transmission. That is, the ESTA which can setthe backoff timer to a small value by a random number obtains thevoluntary transmission right, so that the DCF substantially allows theESTAs attempting to obtain the voluntary transmission right to beequally granted the voluntary transmission right.

As described above, the HCF provides a period during which the ESTA isgranted the TXOP by the HC, and a CP (Contention Period) during whichthe voluntary transmission right is obtained by the above DCF. Forexample, if no CP is provided, the ESTA attempting the transmission ofnormal data, i.e. lower priority data is difficult to be granted theTXOP. Therefore, provision of the CP, as described above, increases theopportunity of securing the voluntary transmission right for the ESTAwhich transmits lower priority data. The amount of time the CP is to beprovided is set as appropriate by the HC, depending on properties ofdata required by the ESTAs.

Here, the following will specifically describe management of the TXOP.As described above, QoS CF-POLL which is transmitted to the ESTA that isto be granted the voluntary transmission right by the HC indicates TXOPLIMIT, which is a duration period of the granted TXOP. That is, an ESTAgranted the TXOP, that is, a TXOP holder is not allowed to transmitbeyond the TXOP LIMIT given. Conversely, the TXOP holder can freelytransmit data frame until the TXOP LIMIT reaches 0.

Further, data frame transmitted during the TXOP is successivelytransmitted at an interval of SIFS (Short Inter Frame Space), which isshorter than the DIFS. Note that, the SIFS corresponds to a minimumframe space in the IEEE 802.11 standards. When acknowledgement needs tobe transmitted in response to a transmitted data frame from a receivingend, upon the reception of a data frame, a receiving station transmitsan acknowledgement frame after the SIFS.

The TXOP holder basically continues data transmission during the TXOPduration. However, it is conceivable that during the TXOP duration, thatis, before the given TXOP LIMIT reaches zero, the TXOP holder has runout of data frame to be transmitted. In this case, the TXOP holdertransmits a frame called “TXOP ending frame” to the receiving station orthe HC after the SIFS following the transmission of the last data frame.When receiving the TXOP ending frame, the HC recognizes that the TXOPgranted to the ESTA, a transmitting end of the TXOP ending frame, hasbeen returned to the HC, and initiates the subsequent sequence, forexample, of transmitting QoS CF-POLL to grant the voluntary transmissionright to another ESTA.

The ESTA other than the TXOP holder is set to reset its NAV value tozero when receiving the TXOP ending frame so that they can initiate theDCF immediately after the end of the TXOP. This allows the ESTA otherthan the TXOP holder to attempt to obtain the voluntary transmissionunder the DCF immediately after the transmission of the TXOP endingframe.

Incidentally, in a wireless communication network, a communicationmedium has a relatively low reliability, and there is a relatively highprobability that the receiving end would fail to correctly receive atransmitted data frame. When such a transmission failure occurs, thereis the possibility that a transmission sequence unexpected by the HC isperformed, instead of a transmission according to the schedulecoordinated by the HC, and management of the voluntary transmissionright is not ensured.

Here, as one example, a schedule coordinated by the HC assumes thatTXOP{circle around (1)} is granted to a first ESTA, and TXOP{circlearound (2)} is then granted to a second ESTA, and thereafter, the CP ofa predetermined period is provided. In this case, when a transmission isperformed according to the schedule coordinated by the HC, a sequence asshown in FIG. 12( a) is performed.

That is, to begin with, the HC transmits QoS CF-POLL{circle around (1)}to the first ESTA. In response to this, the first ESTA transmits dataframes during the TXOP{circle around (1)}. After the end of theTXOP{circle around (1)}, the HC transmits QoS CF-POLL{circle around (2)}to the second ESTA. In response to this, the second ESTA transmits dataframes during the TXOP{circle around (2)}. After the end of theTXOP{circle around (2)}, the CP is started, other ESTA obtains thevoluntary transmission right under the DCF, and DCF{circle around (1)}and DCF{circle around (2)} are performed during the CP.

Here, in the case where the first ESTA transmits TXOP ending framebefore a given TXOP LIMIT reaches zero, a sequence as shown in FIG. 12(b) may be performed.

That is, upon reception of the TXOP ending frame FF transmitted from thefirst ESTA, the HC recognizes that the first ESTA has abandoned theTXOP{circle around (1)}. Then, for grant of the TXOP{circle around (2)}to the second ESTA, which is the subsequent work, the HC transmits theQoS CF-POLL{circle around (2)} to the second ESTA. The second ESTAtransmits data using the TXOP{circle around (2)} which is a period asscheduled. In this case, since the TXOP{circle around (2)} is grantedearlier than it is intended, the CP can be set longer, and the ESTAwhich transmits under the DCF can receive the benefit of enablingtransmission of many sets of data.

In this case, the following problem would occur. Basically, the TXOPending frame transmitted by the TXOP holder is received by the HC.However, it is considered that the HC fails to receive the TXOP endingframe for some reason. In this case, the TXOP holder recognizes that thereturn of a voluntary transmission right has been completed, whereas theHC recognizes that the voluntary transmission right has not beenreturned.

Since the TXOP holder judges that it does not have the voluntarytransmission right, the TXOP holder does not initiate transmission of adata frame until the TXOP holder is newly granted the TXOP or obtainsthe voluntary transmission right under the DCF. On the other hand, sincethe HC judges that the TXOP holder is granted the voluntary transmissionright until the expiry of the TXOP duration given to the TXOP holder,the HC cannot transfer to another sequence such as transmitting QoSCF-POLL to grant the TXOP to another ESTA. That is, as shown in FIG. 12(c), a communication medium is in the idle state during a period of timefrom the transmission of the TXOP ending frame FF by the first ESTA tothe transmission of the QoS CF-POLL{circle around (2)}, which decreasesthe efficiency of bandwidth use. The HCF has been developed for thetransmission of realtime data such as video data, and as many sets ofdata as possible need to be transmitted as quickly as possible, so thatsuch a wasted use of communication bandwidth must be avoided.

Furthermore, in the specifications of the current IEEE 802.11e, the ESTAother than the TXOP holder is set to reset its NAV value when receivingthe TXOP ending frame. Therefore, in the above example, when a period oftime in which no signals are sent out from any communication apparatusesafter the transmission of the TXOP ending frame by the first ESTA lastsfor longer than the DIFS, the ESTA other than the TXOP holder attemptsto obtain a voluntary transmission right under the DCF. Therefore,depending on the circumstances, it is considered that an ESTA which isunexpected by the HC in scheduling would obtain the voluntarytransmission right.

FIG. 12( d) shows a case where after the first ESTA transmits the TXOPending frame FF, the HC fails to receive this TXOP ending frame FF, andimmediately afterward, another ESTA obtains the voluntary transmissionunder the DCF. In this case, transmission under the DCF{circle around(1)}, which is not intended in scheduling coordinated by the HC, isperformed, and the transmission of QoS CF-POLL{circle around (2)} isperformed after the transmission of the DCF{circle around (1)}, that is,with an appreciable delay of the schedule.

Here, in order to let the ESTAs to transmit lower priority data and datawhich are not needed to be transmitted regularly, the HC must prepare agiven period for the CP in advance. If the DCF{circle around (1)}unexpected by the HC is longer than the prepared CP, as a result, theduration of the TXOP{circle around (2)} is set to be shorter than it isintended. That is, the second ESTA which is granted the TXOP{circlearound (2)} is not given the voluntary transmission right of a requestedcondition, which disables a scheduled data transmission. Therefore,there is the possibility that when the second ESTA transmits realtimedata, a trouble including a distorted image would occur at a receivingstation of the realtime data.

As to transmission and reception of QoS CF-POLL, the problem asdescribed above also occurs. That is, when the QoS CF-POLL is correctlyreceived by an ESTA as a destination of QoS CF-POLL, the ESTA as adestination of the QoS CF-POLL recognizes that the TXOP has been grantedto itself and initiates data transmission, so that a wireless signal issent out after the SIFS. However, when the QoS CF-POLL is not correctlyreceived by the ESTA as a destination of the QoS CF-POLL, the ESTA as adestination of the QoS CF-POLL is not supposed to send out a wirelesssignal.

In this case, on the assumption that interruption and transmission by anESTA to which QoS CF-POLL is not addressed does not happen, when the HCdetects that no wireless signals are sent out from any ESTAs in the PIFSperiod after the transmission of the QoS CF-POLL and when, the HC judgesthat QoS CF-POLL frame has not been correctly received by the ESTA as adestination of the QoS CF-POLL frame and retransmits the QoS CF-POLLframe. This retransmission is repeated until the retransmission reachesa certain condition (including a condition where data transmission hasnot been initiated even after the QoS CF-POLL frame is retransmitted fora certain number of times).

At this moment, although none of the ESTAs are supposed to send outwireless signals after the SIFS of the transmission of CF-POLL, there isthe possibility that the HC detects wireless signals for some reason. Inthis case, since the QoS CF-POLL is not correctly received by the ESTA,which is the destination of the QoS CF-POLL, the HC mistakenlyrecognizes that the grant of the TXOP has been completed even though theESTA, which is the destination of the QoS CF-POLL, does not recognizethat the TXOP has been granted.

The possible reasons why the HC mistakenly detects a wireless signalinclude noise, frame transmission under the DCF by the ESTA which hasfailed to receive the QoS CF-POLL and has not set the NAV, and frametransmission by the ESTA belonging to another network in theneighborhood.

In such a case, as in the case with the completion of the TXOP, thereoccurs a period of time during which no data transmission is made fromany devices, which wastes communication bandwidth, depending on thecircumstances, and causes a problem such as unexpected obtaining of thevoluntary transmission right from the HC by the ESTA under DCF.

The present invention is achieved to solve the above problems, and afeature of the present invention is to provide: in a network in whichone network pathway is time-shared by a plurality of communicationapparatuses, a communication management method which allows an accuratemanagement of a voluntary transmission right even in a communicationmedium with low reliability; a communication device; a centralmanagement device; a communication management program; a recordingmedium containing the communication management program; and acommunication system.

SUMMARY OF THE INVENTION

To solve the problems, a communication management method in accordancewith the present invention is a communication management method for acommunication system in which a plurality of communication devices areconnected via a communication network to a central management device formanaging grant of a voluntary transmission right to the plurality ofcommunication devices, and is characterized in that the method includesthe step of: one of the communication devices having been granted thevoluntary transmission right by the central management device,transmitting a transmission right return frame to the central managementdevice to voluntarily return the voluntary transmission right to thecentral management device.

Conventionally, in some cases, the transmission right return frame istransmitted not to the central management device, but to a communicationdevice receiving data from another communication device having avoluntary transmission right. In such cases, it is unknown when thetransmission right return frame will be transmitted; therefore, aftertransmitting the transmission right grant frame, the central managementdevice must perform a reception process on all frames including thoseaddressed to devices other than the central management device, in orderto determine whether the frames are, for example, transmission rightreturn frames. This requires the central management device to perform aprocess with a relatively heavy load, which would result in addedcomplexity and cost in materializing the central management device.

In contrast, according to the above method, the transmission rightreturn frame is transmitted to the central management device.Consequently, according to the method, the central management devicenever misses a transmission right return frame provided that theapparatus performs a reception process on only those frames which areaddressed to the apparatus. This reduces the workload of the centralmanagement device and reduces the complexity, as well as production anddevice costs, in materializing the apparatus.

Another communication management method in accordance with the presentinvention is a communication management method for a communicationsystem in which a plurality of communication devices are connected via acommunication network to a central management device for managing grantof a voluntary transmission right to the plurality of communicationdevices, and is characterized in that the method includes the steps of:the central management device transmitting a transmission right grantframe to one of the communication devices, so as to grant the voluntarytransmission right to the communication device; and the centralmanagement device transmitting the transmission right grant framewithout performing error-correction coding.

A communication device having received a transmission right grant framemay lose the voluntary transmission right to another communicationdevice under DCF, as an example, unless it transmits a frame of somekind in a predetermined period of time after the completion of thereception of the transmission right grant frame. Therefore, thecommunication device having received the transmission right grant frameneeds to determine that the received frame is a transmission right grantframe in a short period of time.

Here, as in the above method, if the transmission right grant frametransmitted from the central management device is not error-correctionencoded, the communication device receiving that frame can determinewhether the received frame is a transmission right grant frame withoutdecoding the error-correction coded frame. This would enable thecommunication device to quickly identify a transmission right grantframe after the reception of the frame. Thus, the communication devicecan initiate a transmission of frames to be transmitted without losingthe voluntary transmission right to another communication device.

A communication management method in accordance with the presentinvention is a communication management method for a communicationsystem in which a plurality of communication devices are connected via acommunication network to a central management device for managing grantof a voluntary transmission right to the plurality of communicationdevices, and is characterized in that the method includes the steps of:the central management device transmitting a transmission right grantframe to one of the communication devices, so as to grant the voluntarytransmission right to the communication device; and the communicationdevice having received the transmission right grant frame alwaysreturning an acknowledgement frame to the central management device in apredetermined sixth period after the reception of the transmission rightgrant frame from the central management device.

Conventionally, in some cases, a transmission right grant frame is usedwhich requires no acknowledgement, and the communication device does notreturn an acknowledgement frame as to the reception of such a frame. Insuch cases, the central management device transmitting a transmissionright grant frame cannot verify an accurate reception of the frame bythe destination communication device.

In contrast, according to the above method, the communication devicehaving received a transmission right grant frame always returns anacknowledgement frame to the transmission right grant frame. Therefore,an acknowledgement frame is returned to the central management devicehaving transmitted a transmission right grant frame. This enables thecentral management device to reliably verify an accurate reception ofthe transmission right grant frame by the destination communicationdevice. Therefore, for example, when the apparatus has failed to verifyan accurate reception of the transmission right grant frame by thedestination communication device, the central management device can takean action: for example, a retransmission of the transmission right grantframe. Thus, the voluntary transmission right grant process is morereliably performed.

Additional features, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a timing chart illustrating a sequence of a TXOP endingprocess implemented by an HC, data transmitting station, data receivingstation, and other ESTAs in a communication system in accordance with anembodiment of the present invention, for cases where a frame is notsuccessfully transmitted/received between the HC and the transmittingstation;

FIG. 2 is an explanatory drawing illustrating, as an example, astructure of a communication system in accordance with the presentembodiment;

FIG. 3 is a timing chart illustrating a sequence of a TXOP endingprocess implemented by the HC, data transmitting station, data receivingstation, and other ESTAs in the communication system, for cases where aframe is successfully transmitted/received between the HC and thetransmitting station;

FIG. 4 is a block diagram depicting functions of the HC and the ESTA;

FIG. 5 is a flow chart showing an outline of a process relating to theending of a TXOP involving the whole communication system;

FIG. 6 is a flow chart showing the flow of a TXOP ending processimplemented by a TXOP holder;

FIG. 7 is a flow chart showing the flow of a TXOP ending processimplemented by an HC;

FIG. 8 is a flow chart showing an outline of a process relating to agrant of a TXOP involving the whole communication system;

FIG. 9 is a flow chart showing the flow of a TXOP grant processimplemented by an HC;

FIG. 10 is a flow chart showing the flow of a TXOP grant processimplemented by a TXOP-granted ESTA;

FIG. 11( a) is an explanatory drawing illustrating a TXOP-granted ESTAtransmitting a set of streams, whereas FIG. 11( b) is an explanatorydrawing illustrating, as an example, how a TXOP duration is splitbetween a set of streams when a TXOP-granted ESTA transmits the set ofstreams; and

FIG. 12( a) through FIG. 12( d) are timing charts showing an example ofdata transmission/reception under a conventional HCF.

DETAILED DESCRIPTION

Referring to FIG. 1 through FIG. 11, the following will describe anembodiment of the present invention.

(Communication System Makeup)

FIG. 2 is an explanatory drawing illustrating, as an example, astructure of a communication system in accordance with the presentembodiment. As shown in the figure, the communication system includes anHC 1 as a central management device and an ESTA-A 2, ESTA-B 3, ESTA-C 4,and ESTA-D 5 as communication devices. All of them are connected via awireless LAN, which is not illustrated. The communication system may beconnected to other communication devices (not illustrated).

The communication system in accordance with the present embodiment maybe implemented in various communication systems. Preferable examplesinclude network systems such as a home-use LAN connecting home electricappliances having a built-in wireless communication function. In thisparticular example, a settop box managing all wireless communicationdevices in a house would be an equivalent to the HC 1 as a centralmanagement device. A DVD player would be an equivalent to thecommunication device as a data transmitting station. A television setwould be an equivalent to the communication device as a data receivingstation. The DVD player would send moving images to the television setunder the management of the settop box.

The wireless LAN used in the communication system in accordance with thepresent embodiment complies with the IEEE 802.11 wireless communicationstandards developed for wireless LAN technology. Also, it is assumedthat an IEEE 802.11e standard be agreed upon. The standard wouldconstitute a part of the IEEE 802.11 wireless communication standardswhich is aimed at the realization of QoS (Quality of Service)communication, for example, real time AV data transfer. The HCF above isimplemented upon this assumption.

Let us suppose that the HC 1 specify the following schedule: First,referring to the arrangement in FIG. 2, the ESTA-A 2 and the ESTA-B 3,for example, receive QoS CF-POLL{circle around (1)} and QoSCF-POLL{circle around (2)} respectively at associated timings. InTXOP{circle around (1)}, the ESTA-A 2 transmits data to a communicationdevice. In TXOP{circle around (2)}, the ESTA-B 3 transmits data to acommunication device. In the CP, the ESTA-C 4 and the ESTA-D 5 transmitsdata to respective communication devices using DCF{circle around (1)}and DCF{circle around (2)}. That is, when the HC 1 can perform normalscheduling, a process is carried out in the above sequence described inreference to FIG. 12( a).

(HC and ESTA Structure)

Now, the following will describe functional arrangements of the HC andthe ESTA, particularly data transmission/reception functions of the HCand the ESTA. Regarding data transmission/reception, the HC and the ESTAhas similar functions; a common functional block diagram will be used inthe following description of both the HC and the ESTA. The HC and theESTA of course differ from each other in other functions, which will notbe described here.

FIG. 4 is a functional block diagram for the HC and the ESTA. As shownin the figure, the HC and the ESTA each include a CPU (CentralProcessing Unit) 6, a frame analyzer 7, a reception buffer 8, a protocolcontroller 9, a signal transmitter/receiver 10, a transmission buffer11, and a frame generator 12.

Assuming this structure, the following will describe (1) an HCtransmitting a QoS CF-POLL to an ESTA to grant a TXOP to the ESTA and(2) the destination ESTA recognizing either a successful reception ofthe QoS CF-POLL or a failed reception of a frame.

First, the CPU 6 in the HC decides granting of a TXOP to an ESTA inscheduling based on information given in advance by all the ESTAs on thenetwork. The CPU 6 in the HC then sends, to the frame generator 12 inthe HC, information on a period in which a TXOP is to be granted, theaddress to which the TXOP is to be granted, and an instruction to startthe generation of a QoS CF-POLL frame.

The frame generator 12 in the HC generates a QoS CF-POLL framecontaining the information on a period in which a TXOP is to be grantedand the information on the address of the ESTA which is to be grantedthe TXOP as the address information. The generator 12 then stores theQoS CF-POLL frame in the transmission buffer 11 in the HC.

Thereafter, the protocol controller 9 in the HC first determines whetherthe HC is currently allowed to make a transmission in accordance withprocedures defined in a protocol (for example, if another communicationdevice is transmitting a wireless signal over a communication medium,the HC must wait for a break in the signal transmission). If it isdetermined that a frame can be transmitted, the protocol controller 9 inthe HC obtains the QoS CF-POLL frame from the transmission buffer 11 inthe HC for conversion to wireless signal. The resultant wireless signalis output to a wireless medium via the signal transmitter/receiver 10 inthe HC.

The wireless signal output from the HC is received in the signaltransmitter/receiver 10 in the ESTA. The received signal is stored inthe reception buffer 8 in the ESTA. The frame analyzer 7 in the ESTAanalyzes the signal stored in the reception buffer 8 in the ESTA in somecases by, for example, error detection. When the signal is determined tobe a correct frame, the frame analyzer 7 in the ESTA reports to the CPU6 in the ESTA the type of the received frame and other informationcontained in the frame. This concludes a reception process.

If the received signal is determined by, for example, error detectionnot to be a correct frame (the signal may contain noise or beincomplete), the frame analyzer 7 in the ESTA reports the failedreception to the CPU 6 in the ESTA, which concludes the receptionprocess.

Having learned of a correct reception of the QoS CF-POLL frame, the CPU6 in the ESTA records a TXOP duration granted to the ESTA and recognizesthe initiation of the TXOP, on the basis of the information, on theperiod in which the TXOP is granted, contained in the received QoSCF-POLL and sent from the frame analyzer 7 in the ESTA. The ESTA thentransmits, for example, data frames which the CPU 6 determines that theESTA should transmit until the granted TXOP duration ends.

Now, referring to FIG. 4, the following will describe (1) a TXOP-grantedESTA (hereinafter, a “TXOP holder”) transmitting a FINAL FRAME(transmission right return frame) to end the TXOP and (2) a destinationHC recognizing either a successful reception of the FINAL FRAME or afailed reception of the frame.

If the TXOP holder has run out of data frame to be transmitted beforethe granted TXOP duration ends, the TXOP holder transmits a frame calleda FINAL FRAME. The FINAL FRAME may be one of several types. The presentembodiment will use a FINAL FRAME called a QUEUE STATE frame as anexample.

The QUEUE STATE frame contains information on the remaining data volumeto be transmitted from the TXOP holder. A single set of information onremaining data volume may be created covering a plurality of streamsindividually. Accordingly, if the TXOP holder is transmitting aplurality of data streams in a granted TXOP duration, a single set ofinformation may show the remaining data volume for each stream.

Next, the stream will be described in more detail. As shown in FIG. 11(a), conventional IEEE 802.11 specifications allow a TXOP-granted ESTA totransmit data to a plurality of ESTAs within a permitted time period. Inother words, two or more streams can be transmitted. Besides, as shownin FIG. 11( b), the ESTA transmitting two or more streams is allowed tospecify the ratio of data transmissions for the stream 1 and the stream2 in the granted TXOP duration.

When this is the case, it would be preferred if the HC is informed ofthe remaining data volume for each stream. A conventional FINAL FRAMEhowever cannot contain information on more than one stream. Therefore,the present embodiment defines the aforementioned new frame called theQUEUE STATE frame as the FINAL FRAME, enabling the HC to be informed ofthe remaining data volume for each stream by the use of the QUEUE STATEframe. Thus, the HC can learn of the remaining data volume for eachstream from each ESTA. Scheduling can be adjusted more accurately.

In other words, once the CPU 6 in the TXOP holder has decided totransmit a QUEUE STATE frame, the CPU 6 sends, to the frame generator 12in the TXOP holder, information on the individual remaining data volumesof all streams to be reported to the HC and an instruction to start thegeneration of a QUEUE STATE frame. The frame generator 12 in the TXOPholder generates a QUEUE STATE frame containing the reported informationon the remaining data volume and the HC address information as theaddress information. The generator 12 then stores the QUEUE STATE framein the transmission buffer 11 in the TXOP holder.

Thereafter, the protocol controller 9 in the TXOP holder firstdetermines whether the TXOP holder is currently allowed to make atransmission in accordance with procedures defined in the protocol. Ifit is determined that data can be transmitted, the protocol controller 9in the TXOP holder obtains the QUEUE STATE frame from the transmissionbuffer 11 in the TXOP holder for conversion to wireless signal. Theresultant wireless signal is output to a wireless medium via the signaltransmitter/receiver 10.

The wireless signal output from the TXOP holder is received in thesignal transmitter/receiver 10 in the HC. The received signal is storedin the reception buffer 8 in the HC. The frame analyzer 7 in the HCanalyzes the signal stored in the reception buffer 5 and reports aresult to the CPU 6 in the HC. Further details of the process will notgiven here because it is similar to the foregoing process implementedwhen the ESTA receives a QoS CF-POLL.

The subsequent part of the description will discuss error correction.Under the HCF of IEEE 802.11, Reed Solomon coding (“RS coding”), anerror correction process, is sometimes used in frame transmission. Theerror correction is an implementation of a certain encoding scheme onframes at the transmitting end. To put it differently, the receiving endcannot analyze the content of the frames unless it decodes encodedsignals. Therefore, the receiving station cannot know the frame type andwhether an acknowledgement is required for the frame, before the RSdecoding completes.

The HCF of IEEE 802.11 specifies that a frame for which anacknowledgement is required is received by an ESTA in a TXOP duration,and the ESTA shall return an acknowledgement frame in an SIFS after thecompletion of the reception. In the present embodiment, acknowledgementframes are transmitted for the FINAL FRAME as a transmission rightreturn frame and the QoS CF-POLL as a transmission right grant frame.The communication device, having received a FINAL FRAME or a QoSCF-POLL, needs to transmit an acknowledgement frame within an SIFS afterthe completion of the frame reception.

The SIFS is relatively short. If a FINAL FRAME or a QoS CF-POLL is RSencoded before transmission, the receiving station, depending on how itimplements RS decoding, may fail to complete RS decoding within an SIFSin which an acknowledgement frame should be transmitted for the receivedframe. When this is the event, the FINAL FRAME or QoS CF-POLL receivingstation can recognize that it needs to return an acknowledgement framefor a received frame, only when the SIFS after the frame reception isover. That is, the receiving station cannot return an acknowledgementframe before the deadline. Therefore, in the present embodiment, RScoding of FINAL FRAMEs and QoS CF-POLLs for transmission is prohibited.

The subsequent part of the description in reference to FIG. 4 willdiscuss (1) a first communication apparatus (ESTA or HC) returning anacknowledgement frame for a received frame, and (2) a secondcommunication apparatus, the addressee of the acknowledgement frame,recognizing either a correct or failed reception of the acknowledgementframe.

First, the frame analyzer 7 in the first communication apparatusanalyzes a signal stored in the reception buffer 8 in a similar mannerto the ESTA receiving a QoS CF-POLL. The analysis confirms correctreception and reports that to the CPU 6 in the first communicationapparatus, and also reports to the CPU 6 whether an acknowledgement isrequired for the received frame.

If the CPU 6 in the first communication apparatus determines from thereceived information that an acknowledgement is required for thereceived frame, the CPU 6 sends information on the receiving end of theacknowledgement frame and an instruction to start the generation of anacknowledgement frame to the frame generator 12 in the firstcommunication apparatus. The frame generator 12 in the firstcommunication apparatus generates an acknowledgement frame containingthe address information reported on the receiving-end communicationapparatus as the address information, and stores the acknowledgementframe in the transmission buffer 11 in the first communicationapparatus.

Thereafter, the protocol controller 9 in the first communicationapparatus first determines whether the first communication apparatus iscurrently allowed to make a transmission in accordance with proceduresdefined in a protocol. If it is determined that data can be transmitted,the protocol controller 9 in the first communication apparatus obtainsthe acknowledgement frame from the transmission buffer 11 in the firstcommunication apparatus for conversion to wireless signal. The resultantwireless signal is output to a wireless medium via the signaltransmitter/receiver 10 in the first communication apparatus.

The wireless signal output from the first communication apparatus isreceived in the signal transmitter/receiver 10 in the secondcommunication apparatus. The received signal is stored in the receptionbuffer 8 in the second communication apparatus. The frame analyzer 7 inthe second communication apparatus analyzes the signal stored in thereception buffer 8 and reports a result to the CPU 6 in the secondcommunication apparatus. Further details of the process in the secondcommunication apparatus will not given here because it is similar to theforegoing process implemented when the ESTA receives a QoS CF-POLL.

(TXOP Ending Process)

The subsequent part of the description will discuss details of an TXOPending. FIG. 3 a timing chart illustrating a sequence of a datatransmission/reception implemented by the HC, data transmitting station,data receiving station, and other ESTAs. Here, the HC is an equivalentto the HC 1 in FIG. 2, and the transmitting station is an equivalent tothe ESTA-A 2 or ESTA-B 3 in FIG. 2. The receiving station is anequivalent to the data receiving ESTA. The other ESTAs are equivalentsto those ESTAs which are basically not involved in a datatransmission/reception in the TXOP.

First, the HC transmits QoS CF-POLL{circle around (1)} to the ESTA-A 2which is the transmitting station. Having successfully received QoSCF-POLL{circle around (1)}, the ESTA-A 2 recognizes that it now has avoluntary transmission right and then continuously transmits data frames{circle around (1)} to {circle around (5)} to an ESTA which is thereceiving station.

Suppose that the ESTA-A 2 has no more frame to be transmitted after thetransmission of data frame {circle around (5)}. In other words, theESTA-A 2 has run out of frames to be transmitted before the TXOP LIMITgiven by QoS CF-POLL{circle around (1)} reaches 0. In thesecircumstances, the ESTA-A 2 transmits a FINAL FRAME as a TXOP endingframe to the HC after an SIFS following the completion of thetransmission of last data frame {circle around (5)} transmitted. The HCreceives the FINAL FRAME and, an SIFS later, transmits an ACK as theacknowledgement frame to the ESTA-A 2 as a TXOP holder.

After the transmission of the FINAL FRAME, the ESTA-A 2 keeps monitoringother stations for a wireless signal output for a period called “PIFS.”The PIFS (Point Coordination Inter Frame Space) is shorter than the DIFSand longer than the SIFS. If having detected a wireless signal outputfrom another station in a PIFS, the ESTA-A 2 stands by until there is abreak in the wireless signal transmission. A break in the wirelesssignal transmission enables the confirmation of the frames transmittedby the wireless signal. Thus, the ESTA-A 2 can know the content of theframes. If the HC has correctly received the FINAL FRAME, an ACK as theacknowledgement frame would be detected for the FINAL FRAME transmittedfrom the HC.

On a detection of an acknowledgement frame ACK transmitted from the HCas predicted, the ESTA-A 2 determines that the HC has correctly receivedthe FINAL FRAME and that the returning of a voluntary transmission rightto the HC has correctly completed. After the determination, the ESTA-A 2operates assuming that it has no voluntary transmission right. That is,the ESTA-A 2 does not transmit a frame until it obtains a voluntarytransmission right either by receiving another QoS CF-POLL addressed toit or through DCF.

The above sequence is implemented if a normal frametransmission/reception is performed between the HC and the transmittingstation when the transmitting station ends a frame transmission in themiddle of a TXOP. The subsequent part of the description will inreference to FIG. 1 discuss a sequence for cases where the frametransmission/reception between the HC and the transmitting station isnot successfully done when the transmitting station ends a frametransmission in the middle of a TXOP.

First, the HC transmits QoS CF-POLL{circle around (1)} to the ESTA-A 2which is the transmitting station. The ESTA-A 2 continuously transmitsdata frames {circle around (1)} to {circle around (5)} to an ESTA whichis the receiving station. Thereafter, the ESTA-A 2 transmits a FINALFRAME as a TXOP ending frame to the HC after an SIFS following thecompletion of the transmission of last data frame {circle around (5)}transmitted. The HC receives the FINAL FRAME and, an SIFS later,transmits an ACK as the acknowledgement frame to the ESTA-A 2 as a TXOPholder.

After the transmission of the FINAL FRAME, the ESTA-A 2 keeps monitoringother stations for a wireless signal output for a PIFS. If a wirelesssignal from another station is detected in a PIFS, the ESTA-A 2 standsby until there is a break in the wireless signal transmission. A breakin the wireless signal transmission enables the confirmation of theframes transmitted by the wireless signal. Thus, the ESTA-A 2 can knowthe content of the frames.

Under these circumstances, three cases are possible: the wireless signalreceived by the ESTA-A 2 is (1) noise, (2) determined to be a wrongframe because of, for example, a CRC (cyclic redundancy checksum) error,or (3) a correct frame, but not the expected acknowledgement frame ACKfor the FINAL FRAME. In these cases, the ESTA-A 2 determines that the HChas not correctly received the FINAL FRAME, and retransmits the FINALFRAME after an SIFS following a break in the wireless signaltransmission. That is, before determining whether or not the FINAL FRAMEbe retransmitted, the ESTA-A 2 stands by until the time when anacknowledgement frame ACK is expected to be transmitted.

Meanwhile, after the complete transmission of the acknowledgement frameACK, the HC keeps monitoring other stations for a wireless signal outputfor a PIFS. If the ESTA-A 2 has correctly received an acknowledgementframe ACK for the FINAL FRAME, it does not retransmit the FINAL FRAME;therefore, no wireless signal is transmitted. In addition,non-TXOP-holder ESTAs have a non-0 NAV specified by the TXOP LIMIT ofthe QoS CF-POLL in a TXOP; therefore, no frame is transmitted in thatperiod. Hence, no communication devices on the network would send awireless signal. If the ESTA-A 2 has failed to correctly receive anacknowledgement frame ACK, the FINAL FRAME is retransmitted; therefore,the ESTA-A 2 sends a wireless signal.

On a failure to detect no wireless signal output from any communicationdevices for the PIFS, the HC determines that the ESTA-A 2 has correctlyreceived an acknowledgement frame ACK and regarded the returning of thevoluntary transmission right as being complete. Then, a next sequence,including a transmission of QoS CF-POLL{circle around (2)}, is startedto grant a TXOP to the ESTA-B 3.

In contrast, on a detection of a wireless signal output from any one ofthe communication devices in the PIFS, the HC determines that the ESTA-A2 has not correctly received an acknowledgement frame ACK and regardsthe returning of the voluntary transmission right as being incomplete.When this is the case, no next sequence is started; the HC stands byagain until a FINAL FRAME is transmitted from the ESTA-A 2.

In other words, before determining whether or not a next sequence bestarted, the HC stands by until the time when the FINAL FRAME isexpected to be retransmitted. Note that beyond the TXOP LIMIT containedin QoS CF-POLL{circle around (1)} transmitted to the ESTA-A 2, theESTA-A 2 always abandons the voluntary transmission right. Hence, the HCstarts a next sequence.

The foregoing description discussed that QoS CF-POLL is transmitted tothe ESTA-B 3 as a next sequence after the HC has transmitted anacknowledgement frame ACK for a FINAL FRAME, that is, the processing ofthe HC granting a transmission right to ESTAs. Alternatively, the HCitself may transmit data frames to communication devices other than theHC. The content of the data frame transmitted may include moving imagesowned by the HC itself.

If no acknowledgement is required for the data frames transmitted fromthe HC, the data frames are continuously transmitted at SIFS intervalsor PIFS intervals. If there arises again a need for the HC to grant atransmission right to an ESTA, a QoS CF-POLL is transmitted after a PIFSfollowing the completion of a transmission of a data frame.

If an acknowledgement is required for the data frames transmitted fromthe HC, a communication device, having received a data frame transmittedfrom the HC, returns a corresponding acknowledgement frame after anSIFS. The HC transmits a next data frame after an SIFS or PIFS followingthe completion of a reception of the acknowledgement frame. If therearises again a need for the HC to grant a transmission right to an ESTA,a QoS CF-POLL is transmitted after a PIFS following the completion of areception of an acknowledgement frame for the data frame.

In this manner, in the present embodiment, no wireless signal istransmitted from any communication devices for less than the DIFS. Thetransmission right granting scheduling in the HC is not disturbed by acommunication device using DCF intercepting and initiating a frametransmission.

(Flow of TXOP Ending Process)

With reference to flow charts in FIG. 5 through FIG. 7, the subsequentpart of the description will discuss the flow of a TXOP ending processimplemented by the communication system in accordance with the presentembodiment. FIG. 5 is a flow chart showing an outline of a processrelating to the ending of a TXOP involving the whole communicationsystem.

First, in step 1 (“S1”), the TXOP holder transmits a FINAL FRAME as aTXOP ending frame to the HC. If the HC has not correctly received theTXOP ending frame (NO in S2), the operation returns to S1 where the TXOPholder retransmits the TXOP ending frame.

In contrast, if the HC has correctly received the TXOP ending frame (YESin S2), the H C transmits an acknowledgement frame after an SIFSfollowing the completion of the reception of the TXOP ending frame (S3).If the TXOP holder has failed to correctly receive the acknowledgementframe (NO in S4), the operation returns to S3 where the HC retransmitsthe acknowledgement frame.

In contrast, if the TXOP holder has correctly received theacknowledgement frame for the FINAL FRAME (YES in S4), the TXOP holderregards the TXOP ending process as being accurately complete (S5),therefore ending the process.

Referring to the flow chart in FIG. 6, the subsequent part of thedescription will discuss the flow of a part of the TXOP ending processwhich is executed by the TXOP holder. First, in S11, the TXOP holdertransmits a TXOP ending frame to the HC. The TXOP holder then determineswhether there is a wireless signal output from any of the ESTAs over thenetwork in a PIFS following the completion of the transmission of theTXOP ending frame (S12). If the TXOP holder detects no wireless signal(NO in S12), the operation returns to S11 where the TXOP ending frame isretransmitted to the HC.

In contrast, if the operation takes the YES path in S12, that is, if awireless signal is detected, the TXOP holder stands by until thewireless signal is transmitted completely (S13). When the transmissionof the wireless signal is completed, the TXOP holder determines whetherthe frame is correct or not by analyzing the signal (S14). A correctframe refers to such a frame that its content is verifiable, that is, aframe free from noise or such a frame that the signal is complete.

If the operation takes the NO path in S14, that is, if the receivedwireless signal is determined not to be a correct frame, the operationreturns to S11 where the TXOP ending frame is retransmitted to the HCafter an SIFS following the completion of the reception of the wirelesssignal (S15).

In contrast, if the operation takes the YES path in S14, that is, if thereceived wireless signal is determined to represent a correct frame, theTXOP holder determines in S16 whether the received frame is anacknowledgement frame for the TXOP ending frame transmitted previously.

If the operation takes the NO path in S16, that is, if the receivedframe is determined not to be an appropriate acknowledgement frame, theoperation returns, after an SIFS following the completion of thereception of the frame, to S11 where the TXOP ending frame isretransmitted to the HC (S15).

In contrast, if the operation takes a YES path in S16, that is, if thereceived frame is determined to be an appropriate acknowledgement frame,the TXOP holder regards the TXOP ending process as being accuratelycomplete (S17), therefore ending the process.

Referring to the flow chart in FIG. 7, the subsequent part of thedescription will discuss the flow of a part of the TXOP ending processwhich is executed by the HC. First, in S21, the HC is standing by untilthe HC receives a TXOP ending frame from the TXOP holder. On thereception of a TXOP ending frame, the operation proceeds to S22 wherethe HC transmits an acknowledgement frame to the TXOP holder after anSIFS following the completion of the reception of the TXOP ending frame.

Then, the HC determines whether there is a wireless signal output fromany of the ESTAs in a PIFS following the completion of the transmissionof the acknowledgement frame (S23). If a wireless signal transmission isconfirmed (YES in S23), the HC determines in S24 whether the receivedwireless signal is a correct frame.

If the operation takes the NO path in S24, that is, if the receivedwireless signal is determined not to be a correct frame, the operationreturns to S21 where the HC stands by to receive a TXOP ending frame. Incontrast, if the operation takes the YES path in S24, that is, if thereceived wireless signal is determined to be a correct frame, the HCdetermines in S25 whether the received frame is a TXOP ending frame.

If the operation takes the NO path in S25, that is, if the receivedframe is determined not to be a TXOP ending frame, the operation returnsto S21 where the HC stands by to receive a TXOP ending frame from theTXOP holder. In contrast, if the operation takes the YES path in S25,that is, if the received frame is determined to be a TXOP ending frame,the HC determines that the TXOP holder has retransmitted the TXOP endingframe because the acknowledgement frame transmitted in S22 failed toaccurately reach the TXOP holder. The operation then returns to S22where the HC retransmits the acknowledgement frame after an SIFSfollowing the completion of the reception of the TXOP ending frame.

In contrast, if the operation takes the NO path in S23, that is, if theHC determines that there has been no wireless signal output from anyESTAs in a PIFS after the completion of the transmission of theacknowledgement frame, the HC regards the TXOP ending process as beingaccurately complete, therefore ending the process.

In the present embodiment, the ESTAs other than the TXOP holderreceiving a FINAL FRAME do not have their NAVs specified to 0. Thisensures that the NAVs of the non-TXOP-holder ESTAs are not 0 in a TXOP.In other words, in ordinary situations, the non-TXOP-holder ESTAs do notinitiate a transmission in a TXOP.

However, if there is an ESTA which has failed to receive a QoS CF-POLLon the network, the NAV of that ESTA is not specified by the TXOP LIMITof the QoS CF-POLL. This creates a possibility that the NAV of the ESTAmay reach 0 even in a TXOP. The ESTA with a 0 NAV due to these reasonsmay intercept a transmission if there occurs no wireless signal outputfrom any communication devices in a frame exchange sequence between theTXOP holder and the HC for the DIFS or longer than the DIFS.

This potential problem is well addressed in the present embodiment:intervals between frames are determined so as to prevent there fromoccurring no wireless signal output from any communication devices forthe DIFS or longer. Thus, chances are low that another ESTA mayintercept in a TXOP ending process.

If a TXOP holder has run out of frames to be transmitted before the TXOPgranted by a QoS CF-POLL ends, the TXOP holder executes the foregoingTXOP ending process. If the HC does not grant a voluntary transmissionright to another ESTA after the TXOP ending process is completed, theESTAs on the network can initiate a transmission under DCF. Note thatunder DCF, it is only ESTAs with a 0 NAV that may obtain a voluntarytransmission right.

By conventional TXOP ending methods, the ESTA having received a TXOPending frame resets its NAV to 0, and therefore can initiate atransmission under DCF immediately after the TXOP ending process iscompleted.

In contrast, in the present embodiment, to further the reliability ofthe TXOP ending process, a rule is provided disallowing the ESTA havingreceived the TXOP ending frame to reset the NAV to 0. The ESTA does notreset the NAV to 0 unless something is done. No ESTAs can initiate atransmission under DCF until the NAV value specified by the QoS CF-POLLat the start of the TXOP is counted down to 0 with time. If there is noscheduled grant of a transmission right from the HC to another ESTA orscheduled voluntary data transmission in that period, there is notransmission from any ESTA on the network, which is a waste ofbandwidth. To address this problem, a mechanism of some form is requiredwhich resets the NAV of an ESTA so that the ESTA can initiate atransmission under DCF.

The mechanism is materialized in the present embodiment by the HCtransmitting a frame of some kind to explicitly permit the initiation ofa transmission under DCF. That frame may be the QoS CF-POLL frame whichthe HC transmitted to the HC itself with the TXOP LIMIT value set to 0.

Suppose that the TXOP holder and HC implemented a TXOP ending process asdescribed above with the HC determining that the TXOP ending process wassuccessfully completed. If there is no scheduled grant of a voluntarytransmission right to another ESTA or scheduled voluntary datatransmission from the HC itself, the HC transmits a QoS CF-POLL frame toitself with the TXOP LIMIT value set to 0. The ESTA having received theQoS CF-POLL frame sets the NAV of the ESTA itself to 0. Thus, the NAV isreset only when the HC explicitly permits the resetting. The HC is notdisturbed by an unexpected ESTA.

Alternatively, the frame transmitted from the HC to explicitly permitthe initiation of a transmission under DCF may be a CF-END frame or aQoS CF-END frame, to name a few. The CF-END frame is originally a frametransmitted to explicitly permit the termination of a period called CFP(Contention Free Period) which is defined in IEEE 802.11. The QoS CF-ENDframe is defined in HCF, IEEE 802.11, but its usage is yet to beclarified in the current version of the specifications.

(TXOP Grant Process)

The foregoing part of the description discussed the TXOP ending process.A TXOP grant process can be implemented by a similar process. Thesubsequent part of the description will describe a TXOP grant process.

Having decided to grant a TXOP to an ESTA, the HC transmits a QoSCF-POLL frame (transmission right grant frame) to the ESTA. The TXOPholder transmits an acknowledgement frame to the HC after an SIFSfollowing the reception of the QoS CF-POLL frame.

After the transmission of the QoS CF-POLL frame, the HC keeps monitoringother stations for a wireless signal output for a PIFS which is shorterthan the DIFS and longer than the SIFS. If having detected a wirelesssignal output from another station in a PIFS, the HC stands by untilthere is a break in the wireless signal transmission.

The HC can know the content of the frames when there is a break in thewireless signal transmission. Under these circumstances, if the TXOPholder has successfully received the QoS CF-POLL frame, anacknowledgement frame would be detected for the FINAL FRAME transmittedby the TXOP holder.

On a detection of an acknowledgement frame transmitted from the TXOPholder as predicted, the HC determines that the TXOP holder hascorrectly received the QoS CF-POLL frame and that the granting of avoluntary transmission right to the TXOP holder has correctly completed.After the determination, the HC operates assuming that it has novoluntary transmission right. That is, the HC does not transmit a frameuntil the granted TXOP duration expires or the TXOP holder transmits aTXOP ending frame.

Meanwhile, if the wireless signal received by the HC does not representa correct frame because it is noise or it contains CRC error, forexample, or does represent a correct frame, but not the expectedacknowledgement frame, the HC determines that the TXOP holder has notcorrectly received the QoS CF-POLL frame, and retransmits the QoSCF-POLL frame after an SIFS following a break in the wireless signaltransmission. That is, the HC waits until the time when a transmissionof an acknowledgement frame is expected, before determining whether theHC will retransmit the QoS CF-POLL frame.

The HC repeatedly retransmits the QoS CF-POLL frame until itsuccessfully receives an acknowledgement frame. If the process wasinfinitely repeated, the HC would fail to grant the voluntarytransmission right to another ESTA as scheduled. To avoid this fromhappening, the retransmission is suspended under certain conditions. Forexample, retransmission may be given up after TXOP, which is originallyscheduled to be granted to the ESTA, following the initiation of thefirst transmission of the QoS CF-POLL. Other conditions may be used.

Meanwhile, the TXOP holder keeps monitoring other stations for awireless signal output in a PIFS after the transmission of theacknowledgement frame. If the HC has correctly received theacknowledgement frame ACK, there are no retransmissions of the QoSCF-POLL frame, and the HC transmits no wireless signals. In addition, inTXOP, non-TXOP-holder ESTAs have a non-zero NAV whose value was given bythe TXOP LIMIT of the QoS CF-POLL, and do not initiate a frametransmission. No devices on the network would transmit a wirelesssignal.

In contrast, if the HC has failed to correctly receive theacknowledgement frame ACK, there occurs a retransmission of the QoSCF-POLL frame, and the HC transmits a wireless signal.

On a failure to detect a wireless signal output from any station in aPIFS, the TXOP holder determines that the HC has correctly received theacknowledgement frame and therefore regarded the granting of thevoluntary transmission right as being complete. The TXOP holder theninitiates a transmission of a frame which the TXOP holder is totransmit.

In contrast, on a detection of a wireless signal output from a stationin a PIFS, the TXOP holder determines that the HC has not correctlyreceived the acknowledgement frame and therefore regarded the grantingof the voluntary transmission right as being incomplete. The TXOP holderwaits for a retransmission of the QoS CF-POLL frame from the HC withoutinitiating a transmission of a frame which the TXOP holder is totransmit. In other words, the TXOP holder waits until the time when aretransmission of the QoS CF-POLL frame is expected, before determiningwhether the TXOP will initiate a transmission of a frame which the TXOPholder is to transmit.

If the TXOP holder has data to be transmitted, data frames aretransmitted in the following sequence. If no acknowledgement is requiredfor the data frames transmitted from the TXOP holder, the data framesare continuously transmitted at SIFS intervals. If there arises again aneed for the TXOP holder to return the transmission right to the HCbecause, for example, the TXOP holder has no more data to betransmitted, the TXOP holder transmits a FINAL FRAME to the HC after anSIFS following the completion of the transmission of the last dataframe.

When an acknowledgement is required for the data frames transmitted fromthe TXOP holder, a communication device, having received a data frametransmitted from the TXOP holder, returns a correspondingacknowledgement frame after an SIFS. The TXOP holder transmits a nextdata frame after an SIFS following the completion of a reception of theacknowledgement frame. If there arises again a need for the TXOP holderto return the transmission right to the HC because, for example, theTXOP holder has no more data to be transmitted, the TXOP holdertransmits a FINAL FRAME to the HC after an SIFS following the completionof the reception of an acknowledgement frame for the data frame.

In contrast, if the TXOP holder has no data to be transmitted, the TXOPtransmits a FINAL FRAME to the HC to return the transmission right tothe HC quickly. Specific steps of implementing the process were alreadydescribed in the foregoing.

In this manner, in the present embodiment, no wireless signal istransmitted from any communication devices for less than the DIFS in atransmission right grant process. The transmission right grantingscheduling in the HC is not disturbed by a communication device usingDCF intercepting and initiating a frame transmission.

(Flow of TXOP Grant Process)

The subsequent part of the description will discuss the flow of the TXOPgrant process in reference to the flow charts in FIG. 8 through FIG. 10.FIG. 8 is a flow chart showing an outline of the TXOP grant processinvolving the whole communication system.

First, in S31, the HC transmits a QoS CF-POLL frame to an ESTA to whicha TXOP will be granted. If the ESTA has failed to correctly receive theQoS CF-POLL frame (NO in S32), the operation returns to S31 where the HCretransmits the QoS CF-POLL frame,

In contrast, if the ESTA to which a TXOP will be granted has correctlyreceived the QoS CF-POLL frame (YES in S32), the ESTA transmits anacknowledgement frame after an SIFS following the completion of thereception of the QoS CF-POLL frame (S33). If the HC has failed tocorrectly receive the acknowledgement frame (NO in S34), the operationreturns to S33 where the ESTA retransmits the acknowledgement frame.

In contrast, if the HC has correctly received the acknowledgement frame(YES in S34), the HC regards the TXOP grant process as being accuratelycomplete (S35), therefor ending the process.

Referring to the flow chart in FIG. 9, the subsequent part of thedescription will discuss a part of the flow of the TXOP grant processwhich is implemented by the HC. First, in S41, the HC transmits a QoSCF-POLL frame to an ESTA to which a TXOP will be granted. The HC thendetermines whether there has been a wireless signal output from any ESTAover the network in a PIFS following the completion of the transmissionof the QoS CF-POLL frame (S42). If no wireless signal is detected (NO inS42), the operation returns to S41 where the HC retransmits the QoSCF-POLL frame to the ESTA.

In contrast, if the operation takes the YES path in S42, that is, if anywireless signal is detected, the HC stands by in S43 until a completionof the wireless signal transmission. Upon the completion of the wirelesssignal transmission, the HC determines whether the frame is correct ornot by analyzing the signal (S44). A correct frame refers to such aframe that its content is verifiable, that is, a frame free from noiseor such a frame that the signal is complete.

If the operation takes the NO path in S44, that is, if the receivedwireless signal is determined not to be a correct frame, the operationreturns to S41 where the HC retransmits the QoS CF-POLL frame to theESTA after an SIFS following the completion of the reception of thewireless signal (S45).

In contrast, if the operation takes the YES path in S44, that is, if thereceived wireless signal is determined to be a correct frame, the HCdetermines in S46 whether the received frame is an acknowledgement framefor the QoS CF-POLL frame transmitted previously.

If the operation takes the NO path in S46, that is, if the receivedframe is determined not to be an appropriate acknowledgement frame, theoperation returns, after an SIFS following the completion of thereception of the frame, to S41 where the HC retransmits the QoS CF-POLLframe to the ESTA to which a TXOP will be granted (S45).

In contrast, if the operation takes the YES path in S46, that is, if thereceived frame is determined to be an appropriate acknowledgement frame,the HC regards the TXOP grant process as being accurately complete(S47), therefore ending the process.

Referring to the flow chart in FIG. 10. First, in S51, the subsequentpart of the description will discuss a part of the flow of the TXOPgrant process which is implemented by the TXOP-granted ESTA. The ESTA isstanding by until the ESTA receives a QoS CF-POLL frame from the HC. Onthe reception of a QoS CF-POLL frame, the operation proceeds to S52where the ESTA transmits an acknowledgement frame to the HC after anSIFS following the completion of the reception of the QoS CF-POLL frame.

Then, the ESTA determines whether there is a wireless signal output fromany of the ESTAs in a PIFS following the completion of the transmissionof the acknowledgement frame (S53). If a wireless signal transmission isconfirmed (YES in S53), the ESTA determines in S54 whether the receivedwireless signal is a correct frame.

If the operation takes the NO path in S54, that is, if the receivedwireless signal is determined not to be a correct frame, the operationreturns to S51 where the ESTA stands by to receive a QoS CF-POLL frame.In contrast, if the operation takes the YES path in S54, that is, if thereceived wireless signal is determined to be a correct frame, the ESTAdetermines in S55 whether the received frame is a QoS CF-POLL frame.

If the operation takes the NO path in S55, that is, if the receivedframe is determined not to be a QoS CF-POLL frame, the operation returnsto S51 where the ESTA stands by to receive a QoS CF-POLL frame from theHC. In contrast, if the operation takes the YES path in S55, that is, ifthe received frame is determined to be a QoS CF-POLL frame, the ESTAdetermines that the HC has retransmitted the QoS CF-POLL frame becausethe acknowledgement frame transmitted in S52 failed to accurately reachthe HC. The operation then returns to S52 where the ESTA retransmits theacknowledgement frame after an SIFS following the completion of thereception of the QoS CF-POLL frame.

In contrast, if the operation takes the NO path in S53, that is, if theESTA determines that there has been no wireless signal output from anyother ESTAs in a PIFS after the completion of the transmission of theacknowledgement frame, the ESTA regards the TXOP grant process as beingaccurately complete, therefore ending the process.

(Frame Interval)

The subsequent part of the description will discuss intervals betweenframes in a frame transmission/reception sequence of the presentembodiment.

A DIFS here is an equivalent to that in the IEEE 802.11 specifications,and defined as a period of time (the thirteenth, third, or eighthperiod) in which the communication device without a voluntarytransmission right may possibly initiate a frame transmission. An SIFSis defined as a period of time (the eleventh, first, or sixth period) inwhich the communication device to which a voluntary transmission rightis to be transferred (HC, TXOP-granted ESTA) transmits anacknowledgement frame after the communication device receives avoluntary transmission right transfer frame (FINAL FRAME (transmissionright return frame), QoS CF-POLL frame (transmission right grantframe)). A PIFS is defined as a period of time (the twelfth, second, orseventh period) in which the communication device with a voluntarytransmission right (TXOP holder or HC) determines whether anacknowledgement frame has been transmitted after the communicationdevice transmitted a transmission right transfer frame (FINAL FRAME, QoSCF-POLL frame). These periods may be an interval of any length providedthat: 13^(th) Period>12^(th) Period>11^(th) Period, 3^(rd) Period>2^(nd)Period>1^(st) Period, or 8^(th) Period>7^(th) Period>6^(th) Period.

Further, an SIFS is defined as a period of time (the fifteenth, fifth,or tenth period) in which the communication device with a voluntarytransmission right retransmits a voluntary transmission right transferframe after the communication device determines that an acknowledgementframe has not been transmitted for the voluntary transmission righttransfer frame. A PIFS is defined as a period of time (the fourteenth,fourth, or ninth period) in which the device to which a voluntarytransmission right is to be transferred determines whether a voluntarytransmission right transfer frame has been retransmitted after thedevice transmitted an acknowledgement frame. These periods may be aninterval of any length provided that: 13^(th) Period>14^(th)Period>15^(th) Period, 3^(rd) Period>4^(th) Period>5^(th) Period, or8^(th) Period>9^(th) Period>10^(th) Period.

(Operations and Effects of the Present Invention)

As described above, a communication management method in accordance withthe present invention is a communication management method for acommunication system in which a plurality of communication devices areconnected via a communication network to a central management device formanaging grant of a voluntary transmission right to the plurality ofcommunication devices, including the step of: one of the communicationdevices having been granted the voluntary transmission right by thecentral management device, transmitting a transmission right returnframe to the central management device to voluntarily return thevoluntary transmission right to the central management device.

Further, a communication management method in accordance with thepresent invention may be such that in the above method, thecommunication device as a transmitting station transmits thetransmission right return frame without being error-correction coding.

According to this method, the central management device having receiveda transmission right return frame can identify that a received frame isa transmission return frame, without decoding the error-correction codedframe. Therefore, in the central management device, it is possible toshorten the length of a period from when the frame is received and whenthe received frame is identified to be the transmission right returnframe, and to initiate frame transmission as the subsequent actionwithout losing the voluntary transmission right to another communicationdevice.

Still further a communication method in accordance with the presentinvention may be such that the central management device always returnsan acknowledgement frame to the communication device in a predeterminedfirst period after reception of the transmission right return frame fromthe communication device.

According to this method, the communication device having transmittedthe transmission right return frame always receives the acknowledgementframe, so that the communication device can reliably verify that thetransmission right return frame has been accurately received by thecentral management device. Therefore, for example, when thecommunication device cannot verify that the transmission right returnframe has been accurately received by the central management device, thecommunication device can take some measures such as retransmission ofthe transmission right return frame, for example, thus ensuring returnprocess of the voluntary transmission right.

Yet further, a communication management method in accordance with thepresent invention may be such that the communication device retransmitsthe transmission right return frame if the communication device does notreceive the acknowledgement frame in response to the transmission rightreturn frame in a second period, longer than the first period, after thetransmission of the transmission right return frame.

If the communication device with the voluntary transmission right canreceive the acknowledgement frame after transmitting the transmissionright return frame, the transmission right return frame is notretransmitted because it can be said that the central management devicehas accurately received the transmission right return frame, that is,the communication device's intention to return the transmission righthas been made known to the central management device.

In the case where the communication device with the voluntarytransmission right cannot receive the acknowledgement frame in a periodof time during which the central management device is expected totransmit the acknowledgement frame after the transmission of thetransmission right return frame, there are the following possible tworeasons.

The first reason is that even though the central management deviceaccurately receives the transmission right return frame and returns theacknowledgement frame, the communication device with the voluntarytransmission right fails to receive the acknowledgement frame inresponse to the transmission right return frame for some reason. Thesecond reason is that since the central management device fails toaccurately receive the transmission right return frame for some reason,the central management device does not return the acknowledgement frame.

In the case of the second reason, while the communication device withthe voluntary transmission right recognizes that the transmission righthas been returned because the communication device has transmitted thetransmission right return frame, the central management devicerecognizes that the transmission right has been not returned yet becausethe central management device has failed to receive the transmissionright return frame. In this case, there occurs a period of time duringwhich no frame transmission is made from any communication devices, thusdecreasing the efficiency of bandwidth use.

In contrast, according to the above method, if the communication devicewith the voluntary transmission right, after transmitting thetransmission right return frame, cannot receive the acknowledgementframe in a period of time during which the central management device isexpected to transmit the acknowledgement frame, the communication deviceretransmits the transmission right return frame. Therefore, thecommunication device with the voluntary transmission right retransmitsthe transmission right return frame until the central management devicecan accurately receive the transmission right return frame. This allowsthe central management device to reliably receive the transmission rightreturn frame, thus enabling the completion of return process of thetransmission right.

Further, a communication management method in accordance with thepresent invention may be such that when having detected no frametransmission from any communication device over a communication networkfor longer than a predetermined third period, the communication deviceis endowed with a right of obtaining the voluntary transmission right;and at the retransmission of the transmission right return frame, thecommunication device with the voluntary transmission right retransmitsthe transmission right return frame in the third period after last frametransmission or frame reception.

According to the above method, if no frame transmission is made from anycommunication devices over the network for a period of time which ismore than the third period (corresponding to the DIFS in the aboveembodiment), all of the communication devices connecting to thiscommunication network can attempt to obtain the voluntary transmissionright. That is, if the communication device with the voluntarytransmission right does not perform frame transmission for a period oftime of more than the third period for some reason, the communicationdevice would lose the voluntary transmission right to anothercommunication device even when the communication device intends to keepholding the voluntary transmission right.

During the return process of the voluntary transmission right, in thecase where a time interval of the frame transmission from when thecommunication device with the voluntary transmission right transmits thetransmission right return frame to when the transmission right returnframe is retransmitted due to a failed reception of the acknowledgementframe or other reason becomes more than the third period, there occursthe possibility that the voluntary transmission right would be lost toanother communication device, and there may be the case where thecommunication device fails to appropriately complete the retransmissionof the transmission right return frame because of another communicationdevice's interruption for its frame transmission.

In contrast, according to the above method, when the communicationdevice having the voluntary transmission right retransmits thetransmission right return frame due to a failed reception of theacknowledgement frame, a failed identification of a type of a framereceived, or other reason, the transmission right return frame isretransmitted in the third period after the last frame transmission orframe reception. This eliminates that a period of time during which noframe transmission is made from any communication devices for more thanthe third period during the return process of the voluntary transmissionright, thus ensuring the retransmission for the transmission rightreturn frame without another communication device's interruption for itstransmission.

Still further, a communication management method in accordance with thepresent invention may be such that the central management devicetransmits to one of the communication devices a transmission right grantframe containing a transmission right grant duration indicating a grantduration of the voluntary transmission right, so as to grant thevoluntary transmission right to the communication device; and thecentral management device avoids voluntarily initiating frametransmission in a period of time from when the transmission of thetransmission right grant frame is completed to when the transmissionright grant duration expires, or in a period of time from when theacknowledgement frame is transmitted in response to the transmissionright return frame to when the central management device detects nowireless signal transmission from any of the communication devices in apredetermined fourth period.

If the communication device having transmitted the transmission rightreturn frame fails to receive, for some reason, the acknowledgementframe which the central management device has transmitted in response tothe transmission right return frame, the transmission right return frameis supposed to be retransmitted. At this point, if the centralmanagement device voluntarily initiates frame transmission immediatelyafter the transmission of the acknowledgement frame, the retransmissionof the transmission right return frame and the voluntary frametransmission by the central management device are performedsimultaneously, so that there is the possibility that a drawbackincluding interference of both frames on the network occurs.

In contrast, according to the above method, the drawback as describedabove does not occur because the central management device, beforeinitiating its frame transmission, verifies that no frame transmissionis initiated from any other communications at the point in time when thepredetermined fourth period expires, in accordance with a predictionthat the transmission right return frame may be retransmitted after thetransmission of the acknowledgement frame in response to thetransmission right return frame.

Yet further, a communication management method in accordance with thepresent invention may be such that the central management devicevoluntarily initiates frame transmission in the third period after thetransmission of the acknowledgement frame in response to thetransmission right return frame.

In the case where the central management device waits for a given periodof time without initiating transmission in accordance with a predictionthat the transmission right return frame may be retransmitted, waitingfor a period of time longer than the third period may cause unexpectedloss of the voluntary transmission right to another communication devicedetecting that the wireless medium is idle for a period of time of morethan the third period. In this case, there occurs a drawback includingdisturbance of the voluntary transmission right granting scheduling bythe central management device.

In contrast, according to the above method, the drawback as describedabove does not occur because the central management device voluntarilyinitiates frame transmission in the third period.

Further, a communication management method in accordance with thepresent invention may be such that if the communication device havingtransmitted the transmission right return frame cannot identify a typeof a frame received immediately after the transmission of thetransmission right return frame, or if the received frame is not theacknowledgement frame in response to the transmission right return framehaving been transmitted previously, the communication device retransmitsthe transmission right return frame in a fifth period, shorter than thefourth period, which is a period of time from when the received frame isreceived to when the central management device voluntarily initiatesframe transmission.

When, in spite of the transmission of the acknowledgement frame by thecentral management device in response to the transmission right returnframe, the communication device having transmitted the transmissionright return frame fails to identify the type of the frame transmittedby the central management device for some reason, the central managementdevice recognizes that the transmission right has been returned whilethe communication device having transmitted the transmission rightreturn frame cannot recognize that return of the transmission right hasbeen completed and retransmits the transmission right return frame. Inthis case, frame transmission by the communication device havingtransmitted the transmission right return frame and frame transmissionby the central management device are performed simultaneously, thuscausing interference of both frames on the network. There is thepossibility that this state would last until the communication devicejudges the expiry of the duration during which the voluntarytransmission right is granted.

In contrast, according to the above method, in such a situation, thecommunication device having transmitted the transmission right returnframe retransmits the transmission right return frame before expiry ofthe fifth period during which the central management device is expectedto initiate voluntary frame transmission. Therefore, the drawback asdescribed above does not occur if the central management device, beforeinitiating the voluntary frame transmission, verifies that other deviceis performing frame transmission and performs the operation such ascancellation of the voluntary frame transmission.

Still further, a communication management method in accordance with thepresent invention is a communication management method for acommunication system in which a plurality of communication devices areconnected via a communication network to a central management device formanaging grant of a voluntary transmission right to the plurality ofcommunication devices, and only the communication device granted thevoluntary transmission right can perform frame transmission, includingthe steps of: the central management device transmitting a transmissionright grant frame to one of the communication devices, so as to grantthe voluntary transmission right to the communication device; and thecommunication device having received the transmission right grant framealways returning an acknowledgement frame to the central managementdevice in a predetermined sixth period after the reception of thetransmission right grant frame from the central management device.

Yet further, a communication management method in accordance with thepresent invention may be such that the central management deviceretransmits the transmission right grant frame if the central managementdevice does not receive the acknowledgement frame in response to thetransmission right grant frame in a seventh period, longer than thesixth period, after the transmission right grant frame is transmitted.

If the central management device receives the acknowledgement frameafter transmitting the transmission right grant frame, the transmissionright grant frame is not retransmitted because it can be said that thecommunication device has accurately received the transmission rightgrant frame, that is, the central management device's intention to grantthe transmission right has been made known to the communication device.

In the case where the central management device fails to receive theacknowledgement frame in a period of time during which the communicationdevice is expected to transmit the acknowledgement frame after thecentral management device transmits the transmission right grant frame,there are the following possible two reasons.

The first reason is that even though the communication device accuratelyreceives the transmission right grant frame and returns theacknowledgement frame, the central management device fails to receivethe acknowledgement frame in response to the transmission right grantframe for some reason. The second reason is that since the communicationdevice fails to accurately receive the transmission right grant framefor some reason, the communication device does not return theacknowledgement frame.

In the case of the second reason, while the central management devicerecognizes that the transmission right has been granted because thecentral management device has transmitted the transmission right grantframe, the communication device recognizes that the transmission righthas been not granted because the communication device has not receivedthe transmission right grant frame. In this case, there occurs a periodof time during which no frame transmission is made from any othercommunication devices, thus decreasing the efficiency of bandwidth use.

In contrast, according to the above method, if the central managementdevice, after transmitting the transmission right grant frame, cannotreceive the acknowledgement frame in a period of time during which thecommunication device is expected to transmit the acknowledgement frame,the central management device retransmits the transmission right grantframe. Therefore, the central management device retransmits thetransmission right grant frame until the communication device canaccurately receive the transmission right grant frame. This allows thecommunication device to reliably receive the transmission right grantframe, thus enabling decrease in the possibility of occurrence of aperiod of time during which no frame transmission is made from anycommunication devices. This increases the efficiency of bandwidth use.

Further, a communication management method in accordance with thepresent invention may be such that when having detected no frametransmission from any of the communication devices for longer than apredetermined eighth period, the communication device is endowed with aright of obtaining the voluntary transmission right; and the centralmanagement device retransmits the transmission right grant frame in theeighth period after the transmission of the transmission right grantframe.

According to the above method, if no frame transmission is made from anycommunication devices over the network for a period of time which ismore than the eighth period (corresponding to the DIFS in the aboveembodiment), all of the communication devices connecting to thiscommunication network can attempt to obtain the voluntary transmissionright. That is, if the communication device with the voluntarytransmission right does not perform frame transmission for a period oftime of more than the eighth period for some reason, the communicationdevice would lose the voluntary transmission right to anothercommunication device even when the communication device intends to keepholding the voluntary transmission right.

During the grant process of the voluntary transmission right, in thecase where a time interval of the frame transmission from when thecentral management device transmits the transmission right grant frameto when the transmission right grant frame is retransmitted due to afailed reception of the acknowledgement frame or other reason becomesmore than the eighth period, there occurs the possibility that thevoluntary transmission right would be lost to another communicationdevice, and there may be the case where the communication device failsto appropriately complete the grant process of the transmission rightbecause of another communication device's interruption for its frametransmission.

In contrast, according to the above method, when the central managementdevice retransmits the transmission right grant frame due to a failedreception of the acknowledgement frame or other reason, the transmissionright grant frame is retransmitted in the eighth period. This eliminatesthat a period of time during which no frame transmission is made fromany communication devices for more than the eighth period during thegrant process of the voluntary transmission right, thus ensuring theretransmission of the transmission right grant frame without anothercommunication device's interruption for its transmission.

Yet further, a communication management method in accordance with thepresent invention may be such that the communication device havingreceived the transmission right grant frame avoids voluntarilyinitiating frame transmission in a period of time from when theacknowledgement frame is transmitted in response to the transmissionright grant frame to when the communication device detects no wirelesssignal transmission from any of the communication devices in apredetermined ninth period.

If the central management device fails to receive, for some reason, theacknowledgement frame which the communication device receiving thetransmission right grant frame have transmitted in response to thetransmission right grant frame, the transmission right grant frame issupposed to be retransmitted. At this point, if the communication devicereceiving the transmission right grant frame initiates frametransmission immediately after the transmission of the acknowledgementframe, the retransmission of the transmission right grant frame and thevoluntary frame transmission by the communication device receiving thetransmission right grant frame are performed simultaneously, so thatthere is the possibility that a drawback including interference of bothframes on the network occurs.

In contrast, according to the above method, the drawback as describedabove does not occur because the communication device receiving thetransmission right grant frame, before initiating its frametransmission, verifies that no frame transmission is initiated from anycommunication devices at the point in time when the predetermined ninthperiod is expired, in accordance with a prediction that the transmissionright return frame may be retransmitted after the transmission of theacknowledgement frame in response to the transmission right grant frame.

Further, a communication management method in accordance with thepresent invention may be such that the communication device havingreceived the transmission right grant frame voluntarily initiates frametransmission in the eighth period after the transmission of theacknowledgement frame in response to the transmission right grant frame.

In the case where the communication device receiving the transmissionright grant frame waits for a given period of time without initiatingtransmission in accordance with a prediction that the transmission rightgrant frame may be retransmitted, waiting for a period of time longerthan the eighth period may cause unexpected loss of the voluntarytransmission right by another communication device detecting that thewireless medium is idle for a period of time of more than the eighthperiod. In this case, there occurs a drawback including disturbance ofthe voluntary transmission right granting scheduling by the centralmanagement device.

In contrast, according to the above method, the drawback as describedabove does not occur because the communication device receiving thetransmission right grant frame voluntarily initiates frame transmissionin the eighth period.

Yet further, a communication management method in accordance with thepresent invention may be such that if the central management devicecannot identify a frame received immediately after the transmission ofthe transmission right grant frame, or if the received frame is not theacknowledgement frame in response to the transmission right grant framehaving transmitted previously, the central management device retransmitsthe transmission right grant frame in a tenth period, longer than theninth period, which is a period of time from when the received frame isreceived to when the communication device having received thetransmission right grant frame voluntarily initiates frame transmission.

When, in spite of the transmission of the acknowledgement frame by thecommunication device receiving the transmission right grant frame inresponse to the transmission right grant frame, the central managementdevice fails to identify a type of the frame transmitted by thecommunication device for some reason, the communication device receivingthe transmission right grant frame recognizes that the transmissionright has been granted while the central management device cannotrecognize that the grant of the transmission right has been completedand retransmits the transmission right grant frame. In this case, frametransmission by the central management device and frame transmission bythe communication device receiving the transmission right grant frameare performed simultaneously, thus causing interference of both frameson the network. There is the possibility that this state would lastuntil the communication device receiving the transmission right grantframe judges the expiry of the duration during which the voluntarytransmission right is granted.

In contrast, according to the above method, in such a situation, thecentral management device retransmits the transmission right grant framebefore expiry of the ninth period during which the communication devicereceiving the transmission right grant frame is expected to initiatevoluntary frame transmission. Therefore, the drawback as described abovedoes not occur when the communication device receiving the transmissionright grant frame, before initiating the voluntary frame transmission,verifies that other device is making frame transmission and performs theoperation such as cancellation of the voluntary frame transmission.

Further, a communication management method in accordance with thepresent invention is a communication management method for use in anetwork system which allows only a communication device with a voluntarytransmission right to initiate a frame exchange sequence and allows acommunication device without the voluntary transmission right totransmit only a frame which is a response to a received frame, includingthe steps of: (a) allowing a communication device to obtain thevoluntary transmission right, the communication device having detectedno frame transmission from any communication device over a communicationnetwork for longer than a thirteenth period; (b) to transfer thevoluntary transmission right to another communication device, thecommunication device with the voluntary transmission right transmittinga voluntary transmission right transfer frame to the other communicationdevice which is a transfer destination of the voluntary transmissionright; (c) the other communication device receiving the voluntarytransmission right transfer frame, transmitting an acknowledgement frameto the communication device which is a transmitting end of the voluntarytransmission right transfer frame after an eleventh period following thereception of the voluntary transmission right transfer frame; and (d)when having detected no frame transmission from any other communicationdevice over the communication network in a twelfth period after thetransmission of the voluntary transmission right transfer frame, thecommunication device having transmitted the voluntary transmission righttransfer frame, retransmitting the voluntary transmission right transferframe in the thirteenth period after the transmission of the voluntarytransmission right transfer frame, wherein: the thirteenth period, theeleventh period, and the twelfth period satisfy the following relation:the thirteenth period>the twelfth period>the eleventh period.

According to the above method, if no frame transmission is performedover the network for a period of time which is more than the thirteenthperiod, all of the communication devices connecting to thiscommunication network can attempt to obtain the voluntary transmissionright. That is, if the communication device having the voluntarytransmission right does not perform frame transmission for a period oftime of more than the thirteenth period for some reason, thecommunication device would lose the voluntary transmission right toanother communication device even when the communication device does notintend to transfer the voluntary transmission right. This is aimed atthe grant of the voluntary transmission right to a communication devicenot explicitly requiring the voluntary transmission right for thepurpose of transmission of low priority data and data not having thenecessity of being transmitted periodically.

Further, if the communication device with the voluntary transmissionright is to transfer the voluntary transmission right to a particularcommunication device, the communication device with the voluntarytransmission right transmits the voluntary transmission right transferframe to the communication device which is a transfer destination. Thatis, the communication device with the voluntary transmission right canspecify the transfer destination of the voluntary transmission right.This enables management of the voluntary transmission right over thecommunication network, thus allowing for transmission scheduling withhigh use efficiency.

Still further, the communication device having received the voluntarytransmission right transfer frame transmits the acknowledgement frame tothe communication device which is a transmitting end of the voluntarytransmission right transfer frame after the eleventh period followingthe reception of the voluntary transmission right transfer frame. Here,the eleventh period is set to be shorter than the thirteenth period.Therefore, the communication device having received the voluntarytransmission right transfer frame can transfer the acknowledgement frameafter the reception of the voluntary transmission right transfer frame,without losing the voluntary transmission right to another communicationdevice.

However, in the communication network with low reliability communicationmedium, such as a wireless communication network, for example, it isconsidered that the voluntary transmission right transfer frame does notaccurately reach to the communication device which is a transferdestination of the voluntary transmission right. In this case, thecommunication device which is a transfer destination of the voluntarytransmission right does not realize that the voluntary transmissionright has been granted to itself, so that the communication device doesnot initiate frame transmission. Then, a period of no frame transmissionexceeds the thirteenth period, thus raising the possibility that acommunication device other than the communication device which is atransfer destination of the voluntary transmission right would obtainthe voluntary transmission right. This disables the transmissionsequence according to a transmission schedule and raises the possibilityof decrease in use efficiency of the communication network.

In contrast, according to the above method, when having detected noframe transmission from any other communication device over thecommunication network in a twelfth period after the transmission of thevoluntary transmission right transfer frame, the communication devicehaving transmitted the voluntary transmission right transfer frameretransmits the voluntary transmission right transfer frame in thethirteenth period after the transmission of the voluntary transmissionright transfer frame. Here, the twelfth period is shorter than thethirteenth period and longer than the eleventh period.

When the voluntary transmission right transfer frame has been accuratelytransmitted to the communication device which is a transfer destinationof the voluntary transmission right, the acknowledgement frame istransmitted after the eleventh period, which is shorter than the twelfthperiod, so that frame transmission is performed over the communicationnetwork in the twelfth period. That is, the communication device havingtransmitted the voluntary transmission right transfer frame can verifythat the voluntary transmission right has been accurately transferred,so that the communication does not retransmit the voluntary transmissionright transfer frame.

On the other hand, when the voluntary transmission right transfer framehas not been accurately transmitted to the communication device which isa transfer destination of the voluntary transmission right, theacknowledgement frame is not transmitted, so that frame transmission isnot performed over the communication network in the twelfth period. Inaddition, there is no frame transmission from any other communicationdevice in the twelfth period, which is shorter than the thirteenthperiod. Therefore, the communication device having transmitted thevoluntary transmission right transfer frame can verify a failed transferof the voluntary transmission right, so that the communication deviceretransmits the voluntary transmission right transfer frame in thethirteenth period. Here, since the voluntary transmission right transferframe is retransmitted in the thirteenth period, the voluntarytransmission right is lost to another communication device.

Therefore, the method as described above, since it is possible to surelytransfer the voluntary transmission right to the communication devicewhich is a transfer destination of the voluntary transmission right, itis also possible to accurately manage the voluntary transmission rightover the communication network with low reliable communication medium.Therefore, for example, for a wireless communication network in whichrealtime data such as video data are transmitted and received, it ispossible to construct a communication network realizing a comfortablecommunication without a trouble such as interrupted display

Further, a communication management method in accordance with thepresent invention may be such that is the above method, when havingdetected no frame transmission from any other communication device overthe communication network in a fourteenth period after the transmissionof the acknowledgement frame, the communication device receiving thevoluntary transmission right transfer frame recognizes that thevoluntary transmission right is transferred to itself, and on the otherhand, when having detected frame transmission from any othercommunication device over the communication network in the fourteenthperiod after the transmission of the acknowledgement frame, thecommunication device receiving the voluntary transmission right transferframe recognizes that the voluntary transmission right is nottransferred to itself; and the thirteenth period and the fourteenthperiod satisfy the following relation: the thirteenth period>thefourteenth period.

After the transmission of the acknowledgement frame by the communicationdevice receiving the voluntary transmission right transfer frame, if theacknowledgement frame is accurately received by the communication devicehaving transmitted the voluntary transmission right transfer frame, datatransmission is not made over the communication network in thefourteenth period which is shorter than the thirteenth period. On theother hand, if the acknowledgement frame is not accurately received bythe communication device having transmitted the voluntary transmissionright transfer frame, the voluntary transmission right transfer frame isretransmitted in the thirteenth period, as described above. Therefore,according to the above method, even in the case of a failed transmissionand reception of the acknowledgement frame, it is possible to surelytransfer the voluntary transmission right without unexpectedly losingthe voluntary transmission right to another communication device.

Yet further, a communication management method in accordance with thepresent invention may be such that in the above method, when havingdetected no frame transmission from any other communication device overthe communication network in a twelfth period after the transmission ofthe voluntary transmission right transfer frame, if judging that theframe transmitted by any other communication device cannot be identifiedor is not the acknowledgement frame, the communication device havingtransmitted the voluntary transmission right transfer frame retransmitsthe voluntary transmission right transfer frame in a fifteenth periodafter the transmission of the frame by any other communication device;and the thirteenth period and the fifteenth period satisfy the followingrelation: the thirteenth period>the fifteenth period.

Frame transmission made over the communication network in the twelfthperiod after the transmission of the voluntary transmission righttransfer frame indicates either transmission of the acknowledgementframe or noise and unscheduled frame transmission. Here, thetransmission of the acknowledgement frame, which indicates that thevoluntary transmission right has been accurately transferred, is not amatter. However, the transmission of noise and unscheduled frameindicates that the voluntary transmission right has not been accuratelytransferred. In view of this, in the above method, when noise andunscheduled frame transmission is made, the voluntary transmission righttransfer frame is retransmitted in the fifteenth period which is shorterthan the thirteenth period. This ensures the transfer of the voluntarytransmission right.

Still further, a communication management method in accordance with thepresent invention may be such that in the above method, the fourteenthperiod is equal to the twelfth period, and the fifteenth period is equalto the eleventh period.

According to the above method, there are three types of periods to beactually managed, thus facilitating the simplification of a periodmanagement process.

Further, a communication management method in accordance with thepresent invention may be such that in the above method, a communicationdevice, other than the communication device which is a transmitting endof the voluntary right transfer frame and the communication device whichis a receiving end of the voluntary right transfer frame, avoids frametransmission in a voluntary transmission right transfer period which iscontained in the voluntary transmission right transfer frame.

Conventionally, when having received the voluntary transmission righttransfer frame, a communication device, other than the communicationdevice which is a transmitting end of the voluntary transmission righttransfer frame and the communication device which is a receiving end ofthe voluntary right transfer frame, makes an attempt at obtaining thevoluntary transmission right if no signal is sent out from anycommunication device for longer than the thirteenth period after thereception of the voluntary transmission right transfer frame. Therefore,in some cases, it is considered that a communication device other thanthe communication device which is a receiving end of the voluntarytransmission right transfer frame obtains the voluntary transmissionright, and there is the possibility that a transmission sequence wouldnot be performed according to the transmission schedule.

In contrast, according to the above method, when having received thevoluntary transmission right transfer frame, the communication devicewhich is a transmitting end of the voluntary transmission right transferframe and the communication device which is a receiving end of thevoluntary right transfer frame checks the voluntary transmission righttransfer period contained in the voluntary transmission right transferframe and avoids frame transmission in this period. Therefore, it ispossible to prevent the communication device other than thecommunication device which is a receiving end of the voluntarytransmission right transfer frame from obtaining the voluntarytransmission right, thus allowing for the realization of a transmissionsequence according to a transmission schedule.

Still further, a communication management method in accordance with thepresent invention may be such that in the above method, when thecommunication device, having received the voluntary transmission righttransfer frame and obtained the voluntary transmission right, has noscheduled frame transmission and no scheduled further transfer of thevoluntary transmission right, the communication device transmits a frameto allow all communication devices connected to the communicationnetwork to make an attempt at obtaining the voluntary transmissionright.

In the aforementioned method, when having received the voluntarytransmission right transfer frame, the communication device other thanthe communication device which is a transmitting end of the voluntarytransmission right transfer frame and the communication device which isa receiving end of the voluntary right transfer frame checks thevoluntary transmission right transfer period contained in the voluntarytransmission right transfer frame and avoids frame transmission in thisperiod. In this case, however, if the communication device havingobtained the voluntary transmission right has no scheduled transmissionand no scheduled further transfer of the voluntary transmission right,neither the communication device having obtained the voluntarytransmission right nor other communication device makes frametransmission. That is the communication network unnecessarily becomesidle.

In contrast, according to the above method, when the communicationdevice having obtained the voluntary transmission right has no scheduledframe transmission and no scheduled further transfer of the voluntarytransmission right, the communication device transmits a frame to allowall communication devices connected to the communication network to makean attempt at obtaining the voluntary transmission right, thus enablingan effective use of the communication bandwidth.

Yet further, a communication management method in accordance with thepresent invention may be such that in the above method, the thirteenthperiod is DIFS in ANSI/IEEE Std 802.11, 1999 Edition, the eleventh andfifteenth periods are SIFS in ANSI/IEEE Std 802.11, 1999 Edition, andthe twelfth and fourteenth periods are PIFS in ANSI/IEEE Std 802.11,1999 Edition.

According to the above method, the above periods can be realized by theperiods standardized in the ANSI/IEEE Std 802.11, 1999 Edition, thuseliminating the need for provision of new standard for a period.Therefore, it is possible to readily introduce the communicationmanagement method in accordance with the present invention.

Further, a communication management method in accordance with thepresent invention may be such that in the above method, thecommunication network includes thereon a central management device formanaging a transmission schedule in the communication network; thecommunication device which is a receiving end of the voluntarytransmission right transfer frame is the central management device; andthe voluntary transmission right transfer frame is a frame indicatingexpiry of a grant duration of the voluntary transmission right.

According to the above method, to the central management device formanaging the transmission schedule in the communication network, theframe indicating expiry of a grant duration of the voluntarytransmission right is transmitted accurately. Therefore, it is possibleto accurately realize the transmission schedule without unexpected frametransmission to the central management device.

Still further, a communication management method in accordance with thepresent invention may be such that in the above method, thecommunication network includes thereon a central management device formanaging a transmission schedule in the communication network; thecommunication device which is a transmitting end of the voluntarytransmission right transfer frame is the central management device; andthe voluntary transmission right transfer frame is a frame indicatinggrant of the voluntary transmission right.

According to the above method, it is possible for the central managementdevice for managing the transmission schedule in the communicationnetwork to ensure transmission of the voluntary transmission right to apredetermined communication device according to the transmissionschedule.

Further, a communication management program in accordance with thepresent invention causes a computer to execute the communicationmanagement method.

By loading the above program in a computer system, it is possible toprovide a user with the above communication management method.

Still further, a recording medium storing a communication managementprogram in accordance with the present invention stores a communicationmanagement program which causes a computer to execute the communicationmanagement program.

By loading the program stored in the recording medium in the computersystem, it is possible to provide a user with the above communicationmanagement method.

CONCLUSION

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

The communication management method in accordance with the presentinvention can be preferably used in network systems such as a home-useLAN connecting home electric appliances having a built-in wirelesscommunication function. More specifically, the communication managementmethod can be applied to a system in which a video data output apparatussuch as DVD player and modem for cable television, and a displayapparatus for displaying video image in accordance with such video data.

1. A communication management method for a communication system in whicha plurality of communication devices are connected via a communicationnetwork to a central management device for managing grant of a voluntarytransmission right to the plurality of communication devices, thecommunication system providing a predetermined first period, apredetermined second period, and a predetermined third period, thepredetermined first period being shorter than the predetermined secondperiod and the predetermined third period, the method comprising:determining one of the communication devices has been granted thevoluntary transmission right by the central management device; andtransmitting a transmission right return frame from the onecommunication device determined as having been granted the voluntarytransmission right to the central management device to voluntarilyreturn the voluntary transmission right to the central managementdevice; when the communication device having detected no frametransmission from any of the communication devices over a communicationnetwork for longer than the predetermined third period, thecommunication device being endowed with a right of obtaining thevoluntary transmission right; and the communication device with thevoluntary transmission right, transmitting the transmission right returnframe after the predetermined first period, which is shorter than thepredetermined third period, following completion of transmission of lastframe.
 2. The communication management method according to claim 1,wherein: the communication device as a transmitting station transmitsthe transmission right return frame without performing error-correctioncoding.
 3. The communication management method according to claim 1,wherein: the central management device always returns an acknowledgementframe to the communication device in the predetermined first periodafter reception of the transmission right return frame from thecommunication device.
 4. The communication management method accordingto claim 3, wherein: the communication device retransmits thetransmission right return frame if the communication device does notreceive the acknowledgement frame in response to the transmission rightreturn frame in the predetermined second period, longer than thepredetermined first period, after the transmission of the transmissionright return frame.
 5. The communication management method according toclaim 4, wherein: when having detected no frame transmission from any ofthe communication devices over a communication network for longer thanthe predetermined third period, the communication device is endowed witha right of obtaining the voluntary transmission right; and thecommunication device with the voluntary transmission right retransmitsthe transmission right return frame in the predetermined third periodafter last frame transmission or frame reception.
 6. The communicationmanagement method according to claim 5, wherein: the central managementdevice transmits to one of the communication devices a transmissionright grant frame containing a transmission right grant durationindicating a grant duration of the voluntary transmission right, so asto grant the voluntary transmission right to the communication device;and the central management device avoids voluntarily initiating frametransmission in a period of time from when the transmission of thetransmission right grant frame is completed to when the transmissionright grant duration expires, or in a period of time from when theacknowledgement frame is transmitted in response to the transmissionright return frame to when the central management device detects nowireless signal transmission from any of the communication devices in apredetermined fourth period.
 7. The communication management methodaccording to claim 5, wherein: the central management device transmitsthe transmission right grant frame or data frame which needs to betransmitted to another communication device when the transmission rightgrant duration expires after transmission of a transmission right grantframe or when the central management device have detected no wirelesssignal transmission from any of the communication devices in apredetermined fourth period after the transmission of theacknowledgement frame in response to the transmission right returnframe.
 8. The communication management method according to claim 6 or 7,wherein: the central management device voluntarily initiates frametransmission in the predetermined third period after the transmission ofthe acknowledgement frame in response to the transmission right returnframe.
 9. The communication management method according to claim 6 or 7,wherein: if the communication device having transmitted the transmissionright return frame cannot identify a type of a frame receivedimmediately after the transmission of the transmission right returnframe, or if the received frame is not the acknowledgement frame inresponse to the transmission right return frame having been transmittedpreviously, the communication device retransmits the transmission rightreturn frame in a predetermined fifth period, shorter than thepredetermined fourth period which is a period of time from when thereceived frame is received to when the central management devicevoluntarily initiates frame transmission.
 10. A communication managementmethod for a communication system in which a plurality of communicationdevices are connected via a communication network to a centralmanagement device for managing grant of a voluntary transmission rightto the plurality of communication devices, the communication systemproviding a predetermined first period, a predetermined second period,and a predetermined third period, the predetermined first period beingshorter than the predetermined second period and the predetermined thirdperiod, the method comprising: the central management devicetransmitting a transmission right grant frame to one of thecommunication devices, so as to grant the voluntary transmission rightto the communication device; when having detected no frame transmissionfrom any of the communication devices for longer than the predeterminedthird period, the communication device is endowed with a right ofobtaining the voluntary transmission right; and the communication devicehaving received the transmission right grant frame always returning anacknowledgement frame to the central management device in thepredetermined first period which is shorter than the predetermined thirdperiod after the reception of the transmission right grant frame fromthe central management device.
 11. The communication management methodaccording to claim 10, wherein: the central management deviceretransmits the transmission right grant frame if the central managementdevice does not receive the acknowledgement frame in response to thetransmission right grant frame in the predetermined second period,longer than the predetermined first period, after the transmission rightgrant frame is transmitted.
 12. The communication management methodaccording to claim 11, wherein: when having detected no frametransmission from any of the communication devices for longer than thepredetermined third period, the communication device is endowed with aright of obtaining the voluntary transmission right; and the centralmanagement device retransmits the transmission right grant frame in thepredetermined third period after last frame transmission or framereception.
 13. The communication management method according to claim12, wherein: the communication device having granted the voluntarytransmission right avoids voluntarily initiating frame transmission in aperiod of time from when the acknowledgement frame is transmitted inresponse to the transmission right grant frame to when the communicationdevice detects no wireless signal transmission from any of thecommunication devices in a predetermined fourth period.
 14. Thecommunication management method according to claim 12, wherein: thecommunication device having granted the voluntary transmission righttransmits the transmission right return frame or data frame which needsto be transmitted to another communication device when the communicationdevice detects no wireless signal transmission from any of thecommunication devices in a predetermined fourth period after thetransmission of the acknowledgement frame in response to thetransmission right grant frame.
 15. The communication management methodaccording to claim 13 or 14, wherein: the communication device havinggranted the voluntary transmission right voluntarily initiates frametransmission in the predetermined third period after the transmission ofthe acknowledgement frame in response to the transmission right grantframe.
 16. The communication management method according to claim 13 or14, wherein: if the central management device cannot identify a type ofa frame received immediately after the transmission of the transmissionright grant frame, or if the received frame is not the acknowledgementframe in response to the transmission right grant frame havingtransmitted previously, the central management device retransmits thetransmission right grant frame in a fifth period which is a period oftime from when the received frame is received to when the communicationdevice having granted the voluntary transmission right voluntarilyinitiates frame transmission.
 17. The communication management methodaccording to claim 1, wherein: the central management device, afterhaving received the transmission right return frame from thecommunication device having granted the voluntary transmission right,transmits to the plurality of communication devices a free-transmissionallowance frame to grant a right of obtaining the voluntary transmissionright.
 18. The communication management method according to claim 17,wherein: the central management device, after having received thetransmission right return frame from the communication device havinggranted the voluntary transmission right, transmits thefree-transmission allowance frame only when the central managementdevice has no further frame transmission.
 19. A communication managementmethod for use in a network system which allows only a communicationdevice with a voluntary transmission right to initiate a frame exchangesequence and allows a communication device without the voluntarytransmission right to transmit only a frame which is a response to areceived frame, the method comprising: allowing a communication deviceto obtain the voluntary transmission right, the communication devicehaving detected no frame transmission from any communication device overa communication network for longer than a first period; to transfer thevoluntary transmission right to another communication device, thecommunication device with the voluntary transmission right transmittinga voluntary transmission right transfer frame to the other communicationdevice which is a transfer destination of the voluntary transmissionright; the other communication device receiving the voluntarytransmission right transfer frame, transmitting an acknowledgement frameto the communication device which is a transmitting end of the voluntarytransmission right transfer frame after a second period following thereception of the voluntary transmission right transfer frame; and whenhaving detected no frame transmission from any other communicationdevice over the communication network in a third period after thetransmission of the voluntary transmission right transfer frame, thecommunication device having transmitted the voluntary transmission righttransfer frame, retransmitting the voluntary transmission right transferframe in the first period after the transmission of the voluntarytransmission right transfer frame, wherein: the first period, the secondperiod, and the third period satisfy the following relation: the firstperiod>the third period>the second period.
 20. The communicationmanagement method according to claim 19, wherein: when having detectedno frame transmission from any other communication device over thecommunication network in a fourth period after the transmission of theacknowledgement frame, the communication device receiving the voluntarytransmission right transfer frame recognizes that the voluntarytransmission right is transferred to itself, and on the other hand, whenhaving detected frame transmission from any other communication deviceover the communication network in the fourth period after thetransmission of the acknowledgement frame, the communication devicereceiving the voluntary transmission right transfer frame recognizesthat the voluntary transmission right is not transferred to itself; andthe first period and the fourth period satisfy the following relation:the first period>the fourth period.
 21. The communication managementmethod according to claim 20, wherein: when having detected no frametransmission from any other communication device over the communicationnetwork in a third period after the transmission of the voluntarytransmission right transfer frame, if judging that the frame transmittedby any other communication device cannot be identified or is not theacknowledgement frame, the communication device having transmitted thevoluntary transmission right transfer frame retransmits the voluntarytransmission right transfer frame in a fifth period after thetransmission of the frame by any other communication device; and thefirst period and the fifth period satisfy the following relation: thefirst period>the fifth period.
 22. The communication management methodaccording to claim 21, wherein: the fourth period is equal to the thirdperiod, and the fifth period is equal to the second period.
 23. Thecommunication management method according to any one of claims 19through 22, wherein: a communication device, other than thecommunication device which is a transmitting end of the voluntary righttransfer frame and the communication device which is a receiving end ofthe voluntary right transfer frame, avoids frame transmission in avoluntary transmission right transfer period which is contained in thevoluntary transmission right transfer frame.
 24. The communicationmanagement method according to claim 23, wherein: when the communicationdevice, having received the voluntary transmission right transfer frameand obtained the voluntary transmission right, has no scheduled frametransmission and no scheduled further transfer of the voluntarytransmission right, the communication device transmits a frame to allowall communication devices connected to the communication network to makean attempt at obtaining the voluntary transmission right.
 25. Thecommunication management method according to claim 21, wherein: thefirst period is DIFS in ANSI/IEEE Std 802.11, 1999 Edition; the secondand fifth periods are SIFS in ANSI/IEEE Std 802.11, 1999 Edition; andthe third and fourth periods are PIFS in ANSI/IEEE Std 802.11, 1999Edition.
 26. The communication management method according to claim 19,wherein: the communication network includes thereon a central managementdevice for managing a transmission schedule in the communicationnetwork; the communication device, a receiving end of the voluntarytransmission right transfer frame, is the central management device; andthe voluntary transmission right transfer frame is a frame indicatingexpiry of a grant duration of the voluntary transmission right.
 27. Thecommunication management method according to claim 19, wherein: thecommunication network includes thereon a central management device formanaging a transmission schedule in the communication network; thecommunication device which is a transmitting end of the voluntarytransmission right transfer frame is the central management device; andthe voluntary transmission right transfer frame is a frame indicatinggrant of the voluntary transmission right.
 28. A communicationmanagement apparatus, the apparatus providing a predetermined firstperiod, a predetermined second period, and a predetermined third period,the predetermined first period being shorter than the predeterminedthird period, comprising: a memory storing a set of instructions; and aprocessor to execute the stored set of instructions to perform themethod comprising: determining one of the communication devices has beengranted the voluntary transmission right by the central managementdevice; and transmitting a transmission right return frame from the onecommunication device determined as having been granted the voluntarytransmission right to the central management device to voluntarilyreturn the voluntary transmission right to the central managementdevice; when the communication device having detected no frametransmission from any of the communication devices over a communicationnetwork for longer than the predetermined third period, thecommunication device being endowed with a right of obtaining thevoluntary transmission right; and the communication device with thevoluntary transmission right, transmitting the transmission right returnframe after the predetermined first period, which is shorter than thepredetermined third period, following completion of transmission of lastframe.
 29. A non-transitory computer readable medium containinginstructions, executed by a processor, for performing a method for acommunication system in which a plurality of communication devices areconnected by a communication network to a central management device formanaging grant of a voluntary transmission right to the plurality ofcommunication devices, the communication system providing apredetermined first period, a predetermined second period, and apredetermined third period, the predetermined first period being shorterthan the predetermined third period, the method comprising: determiningone of the communication devices has been granted the voluntarytransmission right by the central management device; and transmitting atransmission right return frame from the one communication devicedetermined as having been granted the voluntary transmission right tothe central management device to voluntarily return the voluntarytransmission right to the central management device; when thecommunication device having detected no frame transmission from any ofthe communication devices over a communication network for longer thanthe predetermined third period, the communication device being endowedwith a right of obtaining the voluntary transmission right; and thecommunication device with the voluntary transmission right, transmittingthe transmission right return frame after the predetermined firstperiod, which is shorter than the predetermined third period, followingcompletion of transmission of last frame.
 30. A central managementdevice operating in a communication system in which a plurality ofcommunication devices are connected by a communication network to thecentral management device for managing grant of a voluntary transmissionright to the plurality of communication devices, the communicationsystem providing a predetermined first period, a predetermined secondperiod, and a predetermined third period, the predetermined first periodbeing shorter than the predetermined third period, the centralmanagement device comprising: a determination module for determining oneof the communication devices has been granted the voluntary transmissionright by the central management device; and a receiver for receiving atransmission right return frame from the one of the communicationdevices determined as having been granted the voluntary transmissionright to the central management device to voluntarily return thevoluntary transmission right to the central management device; when thecommunication device having detected no frame transmission from any ofthe communication devices over a communication network for longer thanthe predetermined third period, the communication device being endowedwith a right of obtaining the voluntary transmission right; and thecommunication device with the voluntary transmission right, transmittingthe transmission right return frame after the predetermined firstperiod, which is shorter than the predetermined third period, followingcompletion of transmission of last frame.
 31. A central managementdevice, comprising: a memory, storing a set of instructions; and aprocessor, executing the stored set of instructions, to perform themethod for a communication system in which a plurality of communicationdevices are connected via a communication network to the centralmanagement device for managing grant of a voluntary transmission rightto the plurality of communication devices, the communication systemproviding a predetermined first period, a predetermined second period,and a predetermined third period, the predetermined first period beingshorter than the predetermined third period, the method comprising: thecentral management device transmitting a transmission right grant frameto one of the communication devices, so as to grant the voluntarytransmission right to the communication device; when having detected noframe transmission from any of the communication devices for longer thanthe predetermined third period, the communication device is endowed witha right of obtaining the voluntary transmission right; and thecommunication device having received the transmission right grant framealways returning an acknowledgement frame to the central managementdevice in the predetermined first period which is shorter than thepredetermined third period after the reception of the transmission rightgrant frame from the central management device.
 32. A non-transitorycomputer readable medium containing instructions, executed by aprocessor, for performing a method for a communication system in which aplurality of communication devices are connected via a communicationnetwork to a central management device for managing grant of a voluntarytransmission right to the plurality of communication devices, thecommunication system providing a predetermined first period, apredetermined second period, and a predetermined third period, thepredetermined first period being shorter than the predetermined thirdperiod, the method comprising: the central management devicetransmitting a transmission right grant frame to one of thecommunication devices, so as to grant the voluntary transmission rightto the communication device; when having detected no frame transmissionfrom any of the communication devices for longer than the predeterminedthird period, the communication device is endowed with a right ofobtaining the voluntary transmission right; and the communication devicehaving received the transmission right grant frame always returning anacknowledgement frame to the central management device in thepredetermined first period which is shorter than the predetermined thirdperiod after the reception of the transmission right grant frame fromthe central management device.
 33. A communication device comprising: amemory, storing a set of instructions; and a processor, executing thestored set of instructions, to perform the method comprising: allowing acommunication device to obtain the voluntary transmission right, thecommunication device having detected no frame transmission from anycommunication device over a communication network for longer than afirst period; to transfer the voluntary transmission right to anothercommunication device, the communication device with the voluntarytransmission right transmitting a voluntary transmission right transferframe to the other communication device which is a transfer destinationof the voluntary transmission right; the other communication devicereceiving the voluntary transmission right transfer frame, transmittingan acknowledgement frame to the communication device which is atransmitting end of the voluntary transmission right transfer frameafter a second period following the reception of the voluntarytransmission right transfer frame; and when having detected no frametransmission from any other communication device over the communicationnetwork in a third period after the transmission of the voluntarytransmission right transfer frame, the communication device havingtransmitted the voluntary transmission right transfer frame,retransmitting the voluntary transmission right transfer frame in thefirst period after the transmission of the voluntary transmission righttransfer frame, wherein: the first period, the second period, and thethird period satisfy the following relation: the first period>the thirdperiod>the second period.
 34. A non-transitory computer readable mediumcontaining instructions, executed by a processor, for performing themethod of allowing only a communication device with a voluntarytransmission right to initiate a frame exchange sequence and allowing acommunication device without the voluntary transmission right totransmit only a frame which is a response to a received frame, themethod comprising: allowing a communication device to obtain thevoluntary transmission right, the communication device having detectedno frame transmission from any communication device over a communicationnetwork for longer than a first period; to transfer the voluntarytransmission right to another communication device, the communicationdevice with the voluntary transmission right transmitting a voluntarytransmission right transfer frame to the other communication devicewhich is a transfer destination of the voluntary transmission right; theother communication device receiving the voluntary transmission righttransfer frame, transmitting an acknowledgement frame to thecommunication device which is a transmitting end of the voluntarytransmission right transfer frame after a second period following thereception of the voluntary transmission right transfer frame; and whenhaving detected no frame transmission from any other communicationdevice over the communication network in a third period after thetransmission of the voluntary transmission right transfer frame, thecommunication device having transmitted the voluntary transmission righttransfer frame, retransmitting the voluntary transmission right transferframe in the first period after the transmission of the voluntarytransmission right transfer frame, wherein: the first period, the secondperiod, and the third period satisfy the following relation: the firstperiod>the third period>the second period.
 35. The central managementdevice according to claim 30, wherein: the communication device as atransmitting station transmits the transmission right return framewithout performing error-correction coding.
 36. A communication deviceoperating in a communication system in which a plurality ofcommunication devices are connected by a communication network to thecentral management device for managing grant of a voluntary transmissionright to the plurality of communication devices, the communicationsystem providing a predetermined first period, a predetermined secondperiod, and a predetermined third period, the predetermined first periodbeing shorter than the predetermined third period, the centralmanagement device comprising: a determination module for determining oneof the communication devices has been granted the voluntary transmissionright by the central management device; and a receiver for receiving atransmission right return frame from the one of the communicationdevices determined as having been granted the voluntary transmissionright to the central management device to voluntarily return thevoluntary transmission right to the central management device, whereinthe central management device always returns an acknowledgement frame tothe communication device in the predetermined first period afterreception of the transmission right return frame from the communicationdevice; when the communication device having detected no frametransmission from any of the communication devices over a communicationnetwork for longer than the predetermined third period, thecommunication device being endowed with a right of obtaining thevoluntary transmission right; and the communication device with thevoluntary transmission right, transmitting the transmission right returnframe after the predetermined first period, which is shorter than thepredetermined third period, following completion of transmission of lastframe.
 37. The central management device according to claim 36, wherein:the communication device retransmits the transmission right return frameif the communication device does not receive the acknowledgement framein response to the transmission right return frame in the predeterminedsecond period, longer than the predetermined first period, after thetransmission of the transmission right return frame.
 38. The centralmanagement device according to claim 37, wherein: when having detectedno frame transmission from any of the communication devices over acommunication network for longer than the predetermined third period,the communication device is endowed with a right of obtaining thevoluntary transmission right; and the communication device with thevoluntary transmission right retransmits the transmission right returnframe in the predetermined third period after last frame transmission orframe reception.
 39. The central management device according to claim36, wherein: the central management device transmits to one of thecommunication devices a transmission right grant frame containing atransmission right grant duration indicating a grant duration of thevoluntary transmission right, so as to grant the voluntary transmissionright to the communication device; and the central management deviceavoids voluntarily initiating frame transmission in a period of timefrom when the transmission of the transmission right grant frame iscompleted to when the transmission right grant duration expires, or in aperiod of time from when the acknowledgement frame is transmitted inresponse to the transmission right return frame to when the centralmanagement device detects no wireless signal transmission from any ofthe communication devices in a predetermined fourth period.
 40. Thecentral management device according to claim 36, wherein: the centralmanagement device transmits the transmission right grant frame or dataframe which needs to be transmitted to another communication device whenthe transmission right grant duration expires after transmission of atransmission right grant frame or when the central management devicehave detected no wireless signal transmission from any of thecommunication devices in a predetermined fourth period after thetransmission of the acknowledgement frame in response to thetransmission right return frame.
 41. The central management deviceaccording to claim 39 or 40, wherein: the central management devicevoluntarily initiates frame transmission in the predetermined thirdperiod after the transmission of the acknowledgement frame in responseto the transmission right return frame.
 42. The central managementdevice according to claim 39 or 40, wherein: if the communication devicehaving transmitted the transmission right return frame cannot identify atype of a frame received immediately after the transmission of thetransmission right return frame, or if the received frame is not theacknowledgement frame in response to the transmission right return framehaving been transmitted previously, the communication device retransmitsthe transmission right return frame in a predetermined fifth period,shorter than the predetermined fourth period which is a period of timefrom when the received frame is received to when the central managementdevice voluntarily initiates frame transmission.